##### Class 10^{th} Mathematics Gujarat Board Solution

**Exercise 5.1**- a = 3, d = 2 Given a and d for the following A.P., find the following A.P.:…
- a = -3, d = -2 Given a and d for the following A.P., find the following A.P.:…
- a = 100, d = -7 Given a and d for the following A.P., find the following A.P.:…
- a = -100, d = 7 Given a and d for the following A.P., find the following A.P.:…
- a = 1000, d = -100 Given a and d for the following A.P., find the following…
- 5, -5, 5, -5, ... Determine if the following sequences represent an A.P.,…
- 2, 2, 2, 2, ... Determine if the following sequences represent an A.P.,…
- 1, 11, 111, 1111, ... Determine if the following sequences represent an A.P.,…
- 5, 15, 25, 35, 45, ... Determine if the following sequences represent an A.P.,…
- 17, 22, 27, 32, ... Determine if the following sequences represent an A.P.,…
- 101, 99, 97, 95, ... Determine if the following sequences represent an A.P.,…
- 201, 198, 195, 192, ... Determine if the following sequences represent an A.P.,…
- Natural numbers which are consecutive multiples of 5 in increasing order.…
- Natural numbers which are multiples of 3 or 5 in increasing order. Determine if…
- 2, 7, 12, 17, ... Find the nth term of the following A.P.'s:
- 200, 195, 190, 185, ... Find the nth term of the following A.P.'s:…
- 1000, 900, 800, ... Find the nth term of the following A.P.'s:
- 50, 100, 150, 200, ... Find the nth term of the following A.P.'s:…
- 1/2 , 3/2 , 5/2 , 7/2 , 9/2 , l Find the nth term of the following A.P.'s:…
- 1.1, 2.1, 3.1, 4.1, ... Find the nth term of the following A.P.'s:…
- 1.2, 2.3, 3.4, 4.5, ... Find the nth term of the following A.P.'s:…
- 5/3 7/3 , 3 , 11/3 , 13/3 , 5 l Find the nth term of the following A.P.'s:…
- T7 = 12, T12 = 72 Find A.P. if Tn, Tm are as given below:
- T2 = 1, T12 = -9 Find A.P. if Tn, Tm are as given below:
- In an A.P., T3 = 8, T10 = T6 + 20. Find the A.P.
- In an A.P. 5th term is 17 and 9th term exceeds 2nd term by 35. Find the A.P.…
- Can any term of A.P., 12, 17, 22, 27, ... be zero? Why?
- Can any term of A.P., 201, 197, 193, ... be 5? Why?
- Which term of A.P., 8, 11, 14, 17, .. is 272?
- Find the 10th term from end for A.P., 3, 6, 9, 12, ... 300.
- Find the 15th term from end for A.P., 10, 15, 20, 25, 30, ..., 1000.…
- If in an A.P., T7 = 18, T18 = 7, find T101.
- If in an A.P., Tm = n, Tn = m, prove d = -1.

**Exercise 5.2**- 2, 6, 10, 14, ... up to 20 terms Find the sum of the first n terms of the…
- 5, 7, 9, 11, ... upto 30 terms Find the sum of the first n terms of the…
- —10, —12, —14, —16, ... up to 15 terms Find the sum of the first n terms of the…
- 1, 1.5, 2, 2.5, 3, ... upto 16 terms Find the sum of the first n terms of the…
- 1/3 , 4/3 , 7/3 , 10/3 , l up to 18 terms Find the sum of the first n terms of…
- 3 + 6 + 9 + ... + 300 Find the sums indicated below:
- 5 + 10 + 15 + …… + 100 Find the sums indicated below:
- 7 + 12 + 17 + 22 + …… + 102 Find the sums indicated below:
- (-100) + (-92) + (-84) + …… + 92 Find the sums indicated below:
- 25 + 21 + 17 + 13 + …… + (-51) Find the sums indicated below:
- a = 1, d = 2, find S10. For a given A.P. with
- a = 2, d = 3, find S30. For a given A.P. with
- S3 = 9, S7 = 49, find Sn and S10. For a given A.P. with
- T10 = 41, S10 = 320, find Tn, Sn. For a given A.P. with
- S10 = 50, a = 0.5, find d. For a given A.P. with
- S20 = 100, d = —2, find a. For a given A.P. with
- How many terms of A.P., 2, 7, 12, 17, ... add up to 990?
- The first term of finite A.P. is 5, the last term is 45 and the sum is 500. Find…
- If the first term and the last term of a finite A.P. are 5 and 95 respectively…
- The sum of first n terms of an A.P. is 5n — 2n^2 . Find the A.P. i.e. a and d.…
- Find the sum of all three-digit numbers divisible by 3.
- Find the sum of all odd numbers from 5 to 205.
- Which term of A.P. 121, 117, 113, ... is its first negative term? If it is the…

**Exercise 5**- If Tn = 6n + 5, find Sn
- If Sn = n^2 + 2n, find Tn
- If the sum of the first n terms of A, P. 30, 27, 24, 21 … is 120, find the…
- Which term of A.P., 100, 97, 94, 91, will be its first —ve term?
- Find the sum of all 3 digit natural multiples of 6.
- The ratio of the sum of m terms to sum of n terms of an A.P. is m^2/n^2 . Find…
- Sum to first l, m, n terms of A.P. are p, q, r. Prove that p/(m-n) + q/m (n-l) +…
- The ratio of sum to n terms of two A.P. is 8n+1/7n+3 for every n ∈ N. Find the…
- Three numbers in A.P. have the sum 18 and the sum of their squares is 180. Find…
- In a potato race a bucket is placed at the starting point ft is 5 m away from…
- A ladder has rungs 25 cm apart. The rungs decrease uniformly from 60 cm at…
- A man purchased LCD TV for Rs. 32, 500. He paid Rs. 200 initially and…
- In an A.P, T1= 22, Tn = —11, Sn = 66, find n.
- In an A.P. a = 8, Tn= 33, Sn = 123, find d and n.
- If T3 = 8, T7 = 24, then T10 = _____A. —4 B. 28 C. 32 D. 36
- If Sn = 2n^2 + 3n, then d =_____A. 13 B. 4 C. 9 D. —2
- If the sum of the three consecutive terms of A.P. is 48 and the product of the…
- If a = 2 and d = 4, then S20 = _____A. 600 B. 800 C. 78 D. 80
- If 3 + 5 + 7 + 9 + ... upto n terms = 288, then n = ……A. 12 B. 15 C. 16 D. 17…
- Four numbers are in A.P. and their sum is 72 and the largest of them is twice…
- If S1= 2 + 4 + ... + 2n and S2 = 1 + 3 + ... + (2n — 1), then S1: S2 = …..A.…
- For A.P., Sn — 2Sn - 1 + Sn - 2 =…. (n 2)A. 2d B. d C. a D. a + d…
- If Sm = n and Sn = m then Sm + n = ……A. —(m + n) B. 0 C. m + n D. 2m — 2n…
- If T4 = 7 and T7 = 4, then T10 = …..A. 9 B. 11 C. —11 D. 1
- If 2k + 1, 13, 5k — 3 are three consecutive terms of A.P., then k = ……….A. 17…
- (1) + (1 + 1) + (1 + 1 + 1) + ... + (1 + 1 + 1 + ...n — 1 times) =A. (n-1) n/2…
- In the A.P., 5, 7, 9, 11, 13, 15, ... the sixth term which is prime isA. 13 B.…
- For A.P. T18 — T8 = ………A. d B. 10d C. 26d D. 2d
- If for A.P., T25 — T20 = 15 then d =A. 3 B. 5 C. 20 D. 25

**Exercise 5.1**

- a = 3, d = 2 Given a and d for the following A.P., find the following A.P.:…
- a = -3, d = -2 Given a and d for the following A.P., find the following A.P.:…
- a = 100, d = -7 Given a and d for the following A.P., find the following A.P.:…
- a = -100, d = 7 Given a and d for the following A.P., find the following A.P.:…
- a = 1000, d = -100 Given a and d for the following A.P., find the following…
- 5, -5, 5, -5, ... Determine if the following sequences represent an A.P.,…
- 2, 2, 2, 2, ... Determine if the following sequences represent an A.P.,…
- 1, 11, 111, 1111, ... Determine if the following sequences represent an A.P.,…
- 5, 15, 25, 35, 45, ... Determine if the following sequences represent an A.P.,…
- 17, 22, 27, 32, ... Determine if the following sequences represent an A.P.,…
- 101, 99, 97, 95, ... Determine if the following sequences represent an A.P.,…
- 201, 198, 195, 192, ... Determine if the following sequences represent an A.P.,…
- Natural numbers which are consecutive multiples of 5 in increasing order.…
- Natural numbers which are multiples of 3 or 5 in increasing order. Determine if…
- 2, 7, 12, 17, ... Find the nth term of the following A.P.'s:
- 200, 195, 190, 185, ... Find the nth term of the following A.P.'s:…
- 1000, 900, 800, ... Find the nth term of the following A.P.'s:
- 50, 100, 150, 200, ... Find the nth term of the following A.P.'s:…
- 1/2 , 3/2 , 5/2 , 7/2 , 9/2 , l Find the nth term of the following A.P.'s:…
- 1.1, 2.1, 3.1, 4.1, ... Find the nth term of the following A.P.'s:…
- 1.2, 2.3, 3.4, 4.5, ... Find the nth term of the following A.P.'s:…
- 5/3 7/3 , 3 , 11/3 , 13/3 , 5 l Find the nth term of the following A.P.'s:…
- T7 = 12, T12 = 72 Find A.P. if Tn, Tm are as given below:
- T2 = 1, T12 = -9 Find A.P. if Tn, Tm are as given below:
- In an A.P., T3 = 8, T10 = T6 + 20. Find the A.P.
- In an A.P. 5th term is 17 and 9th term exceeds 2nd term by 35. Find the A.P.…
- Can any term of A.P., 12, 17, 22, 27, ... be zero? Why?
- Can any term of A.P., 201, 197, 193, ... be 5? Why?
- Which term of A.P., 8, 11, 14, 17, .. is 272?
- Find the 10th term from end for A.P., 3, 6, 9, 12, ... 300.
- Find the 15th term from end for A.P., 10, 15, 20, 25, 30, ..., 1000.…
- If in an A.P., T7 = 18, T18 = 7, find T101.
- If in an A.P., Tm = n, Tn = m, prove d = -1.

**Exercise 5.2**

- 2, 6, 10, 14, ... up to 20 terms Find the sum of the first n terms of the…
- 5, 7, 9, 11, ... upto 30 terms Find the sum of the first n terms of the…
- —10, —12, —14, —16, ... up to 15 terms Find the sum of the first n terms of the…
- 1, 1.5, 2, 2.5, 3, ... upto 16 terms Find the sum of the first n terms of the…
- 1/3 , 4/3 , 7/3 , 10/3 , l up to 18 terms Find the sum of the first n terms of…
- 3 + 6 + 9 + ... + 300 Find the sums indicated below:
- 5 + 10 + 15 + …… + 100 Find the sums indicated below:
- 7 + 12 + 17 + 22 + …… + 102 Find the sums indicated below:
- (-100) + (-92) + (-84) + …… + 92 Find the sums indicated below:
- 25 + 21 + 17 + 13 + …… + (-51) Find the sums indicated below:
- a = 1, d = 2, find S10. For a given A.P. with
- a = 2, d = 3, find S30. For a given A.P. with
- S3 = 9, S7 = 49, find Sn and S10. For a given A.P. with
- T10 = 41, S10 = 320, find Tn, Sn. For a given A.P. with
- S10 = 50, a = 0.5, find d. For a given A.P. with
- S20 = 100, d = —2, find a. For a given A.P. with
- How many terms of A.P., 2, 7, 12, 17, ... add up to 990?
- The first term of finite A.P. is 5, the last term is 45 and the sum is 500. Find…
- If the first term and the last term of a finite A.P. are 5 and 95 respectively…
- The sum of first n terms of an A.P. is 5n — 2n^2 . Find the A.P. i.e. a and d.…
- Find the sum of all three-digit numbers divisible by 3.
- Find the sum of all odd numbers from 5 to 205.
- Which term of A.P. 121, 117, 113, ... is its first negative term? If it is the…

**Exercise 5**

- If Tn = 6n + 5, find Sn
- If Sn = n^2 + 2n, find Tn
- If the sum of the first n terms of A, P. 30, 27, 24, 21 … is 120, find the…
- Which term of A.P., 100, 97, 94, 91, will be its first —ve term?
- Find the sum of all 3 digit natural multiples of 6.
- The ratio of the sum of m terms to sum of n terms of an A.P. is m^2/n^2 . Find…
- Sum to first l, m, n terms of A.P. are p, q, r. Prove that p/(m-n) + q/m (n-l) +…
- The ratio of sum to n terms of two A.P. is 8n+1/7n+3 for every n ∈ N. Find the…
- Three numbers in A.P. have the sum 18 and the sum of their squares is 180. Find…
- In a potato race a bucket is placed at the starting point ft is 5 m away from…
- A ladder has rungs 25 cm apart. The rungs decrease uniformly from 60 cm at…
- A man purchased LCD TV for Rs. 32, 500. He paid Rs. 200 initially and…
- In an A.P, T1= 22, Tn = —11, Sn = 66, find n.
- In an A.P. a = 8, Tn= 33, Sn = 123, find d and n.
- If T3 = 8, T7 = 24, then T10 = _____A. —4 B. 28 C. 32 D. 36
- If Sn = 2n^2 + 3n, then d =_____A. 13 B. 4 C. 9 D. —2
- If the sum of the three consecutive terms of A.P. is 48 and the product of the…
- If a = 2 and d = 4, then S20 = _____A. 600 B. 800 C. 78 D. 80
- If 3 + 5 + 7 + 9 + ... upto n terms = 288, then n = ……A. 12 B. 15 C. 16 D. 17…
- Four numbers are in A.P. and their sum is 72 and the largest of them is twice…
- If S1= 2 + 4 + ... + 2n and S2 = 1 + 3 + ... + (2n — 1), then S1: S2 = …..A.…
- For A.P., Sn — 2Sn - 1 + Sn - 2 =…. (n 2)A. 2d B. d C. a D. a + d…
- If Sm = n and Sn = m then Sm + n = ……A. —(m + n) B. 0 C. m + n D. 2m — 2n…
- If T4 = 7 and T7 = 4, then T10 = …..A. 9 B. 11 C. —11 D. 1
- If 2k + 1, 13, 5k — 3 are three consecutive terms of A.P., then k = ……….A. 17…
- (1) + (1 + 1) + (1 + 1 + 1) + ... + (1 + 1 + 1 + ...n — 1 times) =A. (n-1) n/2…
- In the A.P., 5, 7, 9, 11, 13, 15, ... the sixth term which is prime isA. 13 B.…
- For A.P. T18 — T8 = ………A. d B. 10d C. 26d D. 2d
- If for A.P., T25 — T20 = 15 then d =A. 3 B. 5 C. 20 D. 25

###### Exercise 5.1

**Question 1.**Given a and d for the following A.P., find the following A.P.:

a = 3, d = 2

**Answer:**Formula Used.

a_{n} = a + (n–1)d

If a = 3, d = 2

Then a_{1} = a + (n–1)d = 3 + (1–1)2 = 3

a_{2} = a + (n–1)d = 3 + (2–1)2 = 3 + 2×1 = 5

a_{3} = a + (n–1)d = 3 + (3–1)2 = 3 + 2×2 = 7

a_{4} = a + (n–1)d = 3 + (4–1)2 = 3 + 2×3 = 9

a_{n} = a + (n–1)d = 3 + (n–1)2 = 2n + 1

∴ The A.P is 3, 5, 7, 9……, 2n + 1

**Question 2.**Given a and d for the following A.P., find the following A.P.:

a = –3, d = –2

**Answer:**Formula Used.

a_{n} = a + (n–1)d

If a = –3, d = –2

Then a_{1} = a + (n–1)d = –3 + (1–1)(–2) = –3

a_{2} = a + (n–1)d = –3 + (2–1)(–2) = –3 + 1×(–2) = –5

a_{3} = a + (n–1)d = –3 + (3–1)(–2) = –3 + 2×(–2) = –7

a_{4} = a + (n–1)d = –3 + (4–1)(–2) = –3 + 3×(–2) = –9

a_{n} = a + (n–1)d = –3 + (n–1)(–2) = –1–2n

∴ The A.P is –3, –5, –7, –9……, –1–2n

**Question 3.**Given a and d for the following A.P., find the following A.P.:

a = 100, d = –7

**Answer:**Formula Used.

a_{n} = a + (n–1)d

If a = 100, d = –7

Then a_{1} = a + (n–1)d = 100 + (1–1)(–7) = 100

a_{2} = a + (n–1)d = 100 + (2–1)(–7) = 100 + (–7)×1 = 93

a_{3} = a + (n–1)d = 100 + (3–1)(–7) = 100 + (–7)×2 = 86

a_{4} = a + (n–1)d = 100 + (4–1)(–7) = 100 + (–7)×3 = 79

a_{n} = a + (n–1)d = 100 + (n–1)(–7) = 107 – 7n

∴ The A.P is 100, 93, 86, 79……107–7n

**Question 4.**Given a and d for the following A.P., find the following A.P.:

a = –100, d = 7

**Answer:**Formula Used.

a_{n} = a + (n–1)d

If a = –100, d = 7

Then a_{1} = a + (n–1)d = –100 + (1–1)7 = –100

a_{2} = a + (n–1)d = –100 + (2–1)7 = –100 + 7×1 = –93

a_{3} = a + (n–1)d = –100 + (3–1)7 = –100 + 7×2 = –86

a_{4} = a + (n–1)d = –100 + (4–1)7 = –100 + 7×3 = –79

a_{n} = a + (n–1)d = –100 + (n–1)7 = 7n–107

∴ The A.P is –100, –93, –86, –79……, 7n–107

**Question 5.**Given a and d for the following A.P., find the following A.P.:

a = 1000, d = –100

**Answer:**Formula Used.

a_{n} = a + (n–1)d

If a = 1000, d = –100

Then;

a_{1} = a + (n–1)d = 1000 + (1–1)(–100) = 1000

a_{2} = a + (n–1)d = 1000 + (2–1)(–100) = 1000 + (–100)×1 = 900

a_{3} = a + (n–1)d = 1000 + (3–1)(–100) = 1000 + (–100)×2 = 800

a_{4} = a + (n–1)d = 1000 + (4–1)(–100) = 1000 + (–100)×3 = 700

a_{n} = a + (n–1)d = 1000 + (n–1)(–100) = 1100–100n

∴ The A.P is 1000, 900, 800, 700……, 1100–100n

**Question 6.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

5, –5, 5, –5, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

In the above sequence,

a = 5;

d_{1} = a_{2}–a_{1} = (–5)–5 = –10

d_{2} = a_{3}–a_{2} = 5–(–5) = 10

d_{3} = a_{4}–a_{3} = (–5)–5 = –10

⇒ As in A.P the difference between the 2 terms is always constant

But the difference in sequence is not the same.

∴ The above sequence is not A.P

**Question 7.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

2, 2, 2, 2, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 2;

d_{1} = a_{2}–a_{1} = 2–2 = 0

d_{2} = a_{3}–a_{2} = 2–2 = 0

d_{3} = a_{4}–a_{3} = 2–2 = 0

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence comes to be 0.

∴ The above sequence is not an A.P

**Question 8.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

1, 11, 111, 1111, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

In the above sequence,

a = 1;

d_{1} = a_{2}–a_{1} = 11–1 = 10

d_{2} = a_{3}–a_{2} = 111–11 = 100

d_{3} = a_{4}–a_{3} = 1111–111 = 1000

⇒ As in A.P the difference between the 2 terms is always constant

But the difference in sequence is not the same.

∴ The above sequence is not A.P

**Question 9.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

5, 15, 25, 35, 45, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 5;

d_{1} = a_{2}–a_{1} = 15–5 = 10

d_{2} = a_{3}–a_{2} = 25–15 = 10

d_{3} = a_{4}–a_{3} = 35–25 = 10

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be 10.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 5 + (n–1)10

= 5 + 10n–10

= –5 + 10n

**Question 10.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

17, 22, 27, 32, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 17;

d_{1} = a_{2}–a_{1} = 22–17 = 5

d_{2} = a_{3}–a_{2} = 27–22 = 5

d_{3} = a_{4}–a_{3} = 32–27 = 5

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be 5.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 17 + (n–1)5

= 17 + 5n–5

= 12 + 5n

**Question 11.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

101, 99, 97, 95, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 101;

d_{1} = a_{2}–a_{1} = 99–101 = –2

d_{2} = a_{3}–a_{2} = 97–99 = –2

d_{3} = a_{4}–a_{3} = 95–97 = –2

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be (–2).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 101 + (n–1)(–2)

= 101–2n + 2

= 103–2n

**Question 12.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

201, 198, 195, 192, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 201;

d_{1} = a_{2}–a_{1} = 198–201 = –3

d_{2} = a_{3}–a_{2} = 195–198 = –3

d_{3} = a_{4}–a_{3} = 192–195 = –3

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be (–3).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 201 + (n–1)(–3)

= 201–3n + 3

= 204–3n

**Question 13.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

Natural numbers which are consecutive multiples of 5 in increasing order.

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

The sequence of natural numbers which are consecutive multiples of 5

Is 5, 10, 15, 20 ……

In the above sequence,

a = 5;

d_{1} = a_{2}–a_{1} = 10–5 = 5

d_{2} = a_{3}–a_{2} = 15–10 = 5

d_{3} = a_{4}–a_{3} = 20–15 = 5

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be 5.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 5 + (n–1)5

= 5 + 5n–5

= 5n

**Question 14.**Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

Natural numbers which are multiples of 3 or 5 in increasing order.

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

The sequence of natural numbers which are multiple of 3 or 5

in increasing order

Is 3, 5, 6, 9, 10 ……

In the above sequence,

a = 3;

d_{1} = a_{2}–a_{1} = 5–3 = 2

d_{2} = a_{3}–a_{2} = 6–5 = 1

d_{3} = a_{4}–a_{3} = 9–6 = 3

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is not same .

∴ The above sequence is not A.P

**Question 15.**Find the nth term of the following A.P.'s:

2, 7, 12, 17, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 2;

d_{1} = a_{2}–a_{1} = 7–2 = 5

d_{2} = a_{3}–a_{2} = 12–7 = 5

d_{3} = a_{4}–a_{3} = 17–12 = 5

The difference in sequence is same and comes to be 5 .

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 2 + (n–1)(5)

= 2 + 5n–5

= –3 + 5n

**Question 16.**Find the nth term of the following A.P.'s:

200, 195, 190, 185, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 200;

d_{1} = a_{2}–a_{1} = 195–200 = –5

d_{2} = a_{3}–a_{2} = 190–195 = –5

d_{3} = a_{4}–a_{3} = 185–190 = –5

The difference in sequence is same and comes to be (–5).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 200 + (n–1)(–5)

= 200–5n + 5

= 205–5n

**Question 17.**Find the nth term of the following A.P.'s:

1000, 900, 800, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 1000;

d_{1} = a_{2}–a_{1} = 900–1000 = –100

d_{2} = a_{3}–a_{2} = 800–900 = –100

The difference in sequence is same and comes to be (–100).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 1000 + (n–1)(–100)

= 1000–100n + 100

= 1100–100n

**Question 18.**Find the nth term of the following A.P.'s:

50, 100, 150, 200, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 50;

d_{1} = a_{2}–a_{1} = 100–50 = 50

d_{2} = a_{3}–a_{2} = 150–100 = 50

d_{3} = a_{4}–a_{3} = 200–150 = 50

The difference in sequence is same and comes to be 50.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 50 + (n–1)(50)

= 50 + 50n–50

= 50n

**Question 19.**

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = ;

d_{1} = a_{2}–a_{1} = = 1

d_{2} = a_{3}–a_{2} = = 1

d_{3} = a_{4}–a_{3} = = 1

The difference in sequence is same and comes to be (1).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = + (n–1)(1)

= + n–1

= n–

**Question 20.**Find the nth term of the following A.P.'s:

1.1, 2.1, 3.1, 4.1, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 1.1;

d_{1} = a_{2}–a_{1} = 2.1–1.1 = 1

d_{2} = a_{3}–a_{2} = 3.1–2.1 = 1

d_{3} = a_{4}–a_{3} = 4.1–3.1 = 1

The difference in sequence is same and comes to be (1).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 1.1 + (n–1)(1)

= 1.1 + n–1

= 0.1 + n

**Question 21.**Find the nth term of the following A.P.'s:

1.2, 2.3, 3.4, 4.5, ...

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 1.2;

d_{1} = a_{2}–a_{1} = 2.3–1.2 = 1.1

d_{2} = a_{3}–a_{2} = 3.4–2.3 = 1.1

d_{3} = a_{4}–a_{3} = 4.5–3.4 = 1.1

The difference in sequence is same and comes to be (1.1).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 1.2 + (n–1)(1.1)

= 1.2 + 1.1n–1.1

= 0.1 + 1.1n

**Question 22.**Find the nth term of the following A.P.'s:

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = ;

d_{1} = a_{2}–a_{1} = =

d_{2} = a_{3}–a_{2} = =

d_{3} = a_{4}–a_{3} = =

The difference in sequence is same and comes to be ().

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = + (n–1)()

=

= 1 + n

**Question 23.**Find A.P. if Tn, Tm are as given below:

T_{7} = 12, T_{12} = 72

**Answer:**Formula Used.

a_{n} = a + (n–1)d

a_{7} = a + (7–1)d

a_{7} = a + 6d

If 7^{th} term of A.P is given as 12

Then,

a + 6d = 12

we get a = 12–6d ......eq 1

a_{12} = a + (12–1)d

a_{12} = a + 11d

If 12^{th} term of A.P is given as 72

Then,

a + 11d = 72

we get a = 72–11d ......eq 2

Equating both eq 1 and eq 2

We get ;

12–6d = 72–11d

11d–6d = 72–12

5d = 60

d = = 12

Putting d in eq 1 we get ;

a = 12–6×12

= 12–72 = –60

As a = –60 and d = 12

Then;

a_{1} = a + (n–1)d = –60 + (1–1)(12) = –60

a_{2} = a + (n–1)d = –60 + (2–1)(12) = –60 + (12)×1 = –48

a_{3} = a + (n–1)d = –60 + (3–1)(12) = –60 + (12)×2 = –36

a_{4} = a + (n–1)d = –60 + (4–1)(12) = –60 + (12)×3 = –24

a_{n} = a + (n–1)d = –60 + (n–1)(12) = 12n – 72

∴ The A.P is –60, –48, –36, –24……, 12n – 72

**Question 24.**Find A.P. if Tn, Tm are as given below:

T_{2} = 1, T_{12} = –9

**Answer:**Formula Used.

a_{n} = a + (n–1)d

a_{2} = a + (2–1)d

a_{2} = a + d

If 2^{nd} term of A.P is given as 1

Then,

a + d = 1

we get a = 1–d ......eq 1

a_{12} = a + (12–1)d

a_{12} = a + 11d

If 12^{th} term of A.P is given as –9

Then,

a + 11d = –9

we get a = –9–11d ......eq 2

Equating both eq 1 and eq 2

We get ;

1–d = –9–11d

11d–d = –9–1

10d = –10

d = = –1

Putting d in eq 1 we get ;

a = 1–(–1)

= 2

As a = 2 and d = –1

Then;

a_{1} = a + (n–1)d = 2 + (1–1)(–1) = 2

a_{2} = a + (n–1)d = 2 + (2–1)(–1) = 2 + (–1)×1 = 2–1 = 1

a_{3} = a + (n–1)d = 2 + (3–1)(–1) = 2 + (–1)×2 = 2–2 = 0

a_{4} = a + (n–1)d = 2 + (4–1)(–1) = 2 + (–1)×3 = 2–3 = –1

a_{n} = a + (n–1)d = 2 + (n–1)(–1) = 2 + 1–n = 3–n

∴ The A.P is 2, 1, 0, –1……, 3–n

**Question 25.**In an A.P., T_{3} = 8, T_{10} = T_{6} + 20. Find the A.P.

**Answer:**Formula Used.

a_{n} = a + (n–1)d

a_{3} = a + (3–1)d

a_{3} = a + 2d

If 3^{rd} term of A.P is given as 8

Then,

a + 2d = 8

we get a = 8–2d ......eq 1

a_{10} = a + (10–1)d

a_{10} = a + 9d

If 10^{th} term of A.P is given as 20 + 6^{th} term

Then,

a_{6} = a + (6–1)d

= a + 5d

a + 9d = 20 + [a + 5d]

we get

a–a + 9d–5d = 20

4d = 20

d = = 5 ......eq 2

Putting d in eq 1 we get ;

a = 8–(2×5)

= 8–10 = –2

As a = –2 and d = 5

Then;

a_{1} = a + (n–1)d = –2 + (1–1)(5) = –2

a_{2} = a + (n–1)d = –2 + (2–1)(5) = –2 + (5)×1 = –2 + 5 = 3

a_{3} = a + (n–1)d = –2 + (3–1)(5) = –2 + (5)×2 = –2 + 10 = 8

a_{4} = a + (n–1)d = –2 + (4–1)(5) = –2 + (5)×3 = –2 + 15 = 13

a_{n} = a + (n–1)d = –2 + (n–1)(5) = –2–5 + 5n = –7 + 5n

∴ The A.P is –2, 3, 8, 13, ……, 5n–7

**Question 26.**In an A.P. 5th term is 17 and 9th term exceeds 2nd term by 35. Find the A.P.

**Answer:**Formula Used.

a_{n} = a + (n–1)d

a_{5} = a + (5–1)d

a_{5} = a + 4d

If 5^{th} term of A.P is given as 17

Then,

a + 4d = 17

we get a = 17–4d ......eq 1

a_{9} = a + (9–1)d

a_{9} = a + 8d

If 9^{th} term of A.P is given as 35 + 2^{nd} term

Then,

a_{2} = a + (1)d

= a + d

a + 8d = 35 + [a + d]

we get

a–a + 8d–d = 35

7d = 35

d = = 5 ......eq 2

Putting d in eq 1 we get ;

a = 17–(4×5)

= 17–20 = –3

As a = –3 and d = 5

Then;

a_{1} = a + (n–1)d = –3 + (1–1)(5) = –3

a_{2} = a + (n–1)d = –3 + (2–1)(5) = –3 + (5)×1 = –3 + 5 = 2

a_{3} = a + (n–1)d = –3 + (3–1)(5) = –3 + (5)×2 = –3 + 10 = 7

a_{4} = a + (n–1)d = –3 + (4–1)(5) = –3 + (5)×3 = –3 + 15 = 12

a_{n} = a + (n–1)d = –3 + (n–1)(5) = –3 + 5n–5 = 5n–8

∴ The A.P is –3, 2, 7, 12, ……, 5n–8

**Question 27.**Can any term of A.P., 12, 17, 22, 27, ... be zero? Why?

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 12;

d_{1} = a_{2}–a_{1} = 17–12 = 5

d_{2} = a_{3}–a_{2} = 22–17 = 5

d_{3} = a_{4}–a_{3} = 27–22 = 5

The difference in sequence is same and comes to be (5).

For any term of A.P to be 0

a_{n} = a + (n–1)d = 0

a_{n} = a + (n–1)d = 12 + (n–1)(5)

= 12 + 5n–5

= 7 + 5n

7 + 5n = 0

5n = –7

n =

∴ The number of terms cannot be negative

Hence, the no term of A.P can be 0

**Question 28.**Can any term of A.P., 201, 197, 193, ... be 5? Why?

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 201;

d_{1} = a_{2}–a_{1} = 197–201 = –4

d_{2} = a_{3}–a_{2} = 193–197 = –4

The difference in sequence is same and comes to be (–4).

For any term of A.P to be 5

a_{n} = a + (n–1)d = 5

a_{n} = a + (n–1)d = 201 + (n–1)(–4)

5 = 201–4n + 4

5 = 205–4n

–4n = –205 + 5

n = = 50

∴ The 50^{th} term of A.P is 5

**Question 29.**Which term of A.P., 8, 11, 14, 17, .. is 272?

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 8;

d_{1} = a_{2}–a_{1} = 11–8 = 3

d_{2} = a_{3}–a_{2} = 14–11 = 3

d_{3} = a_{4}–a_{3} = 17–14 = 3

The difference in sequence is same and comes to be (3).

For any term of A.P to be 272

a_{n} = a + (n–1)d = 272

a_{n} = a + (n–1)d = 8 + (n–1)(3)

= 8 + 3n–3

= 5 + 3n

5 + 3n = 272

3n = 272–5 = 267

n = = 89

∴ The 89^{th} term of A.P has value 272

**Question 30.**Find the 10th term from end for A.P., 3, 6, 9, 12, ... 300.

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 3;

d_{1} = a_{2}–a_{1} = 6–3 = 3

d_{2} = a_{3}–a_{2} = 9–6 = 3

d_{3} = a_{4}–a_{3} = 12–9 = 3

The difference in sequence is same and comes to be (3).

As the last term is 300

a_{n} = a + (n–1)d

300 = 3 + (n–1)3

300 = 3 + 3n–3 = 3n

n = = 100^{th} term

10^{th} term from the end is

100 + 1–10 = 91^{st} term

∵ while counting, last term is also counted

a_{n} = a + (n–1)d

a_{91} = 3 + (91–1)3

= 3 + 90×3

= 273

**Question 31.**Find the 15th term from end for A.P., 10, 15, 20, 25, 30, ..., 1000.

**Answer:**Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 10;

d_{1} = a_{2}–a_{1} = 15–10 = 5

d_{2} = a_{3}–a_{2} = 20–15 = 5

d_{3} = a_{4}–a_{3} = 25–20 = 5

The difference in sequence is same and comes to be (5).

As the last term is 1000

a_{n} = a + (n–1)d

1000 = 10 + (n–1)5

1000 = 10 + 5n–5 = 5 + 5n

5n = 995

n = = 199^{th} term

15 term from the end will be

199 + 1–15 = 185^{th} term

a_{n} = a + (n–1)d

a_{185} = 10 + (185–1)5

= 10 + 184×5

= 10 + 920

= 930

**Question 32.**If in an A.P., T_{7} = 18, T_{18} = 7, find T_{101}.

**Answer:**Formula Used.

a_{n} = a + (n–1)d

a_{7} = a + (7–1)d

a_{7} = a + 6d

If 7^{th} term of A.P is given as 18

Then,

a + 6d = 18

we get a = 18–6d ......eq 1

a_{18} = a + (18–1)d

a_{18} = a + 17d

If 18^{th} term of A.P is given as 7

Then,

a + 17d = 7

we get a = 7–17d ......eq 2

Equating both eq 1 and eq 2

We get ;

18–6d = 7–17d

17d–6d = 7–18

11d = –11

d = = –1

Putting d in eq 1 we get ;

a = 18–6×(–1)

= 18 + 6 = 24

For 101^{st} term

a_{n} = a + (n–1)d

a_{101} = 24 + (101–1)(–1)

= 24–100

= –76

**Question 33.**If in an A.P., T_{m} = n, T_{n} = m, prove d = –1.

**Answer:**Formula Used.

a_{n} = a + (n–1)d

a_{m} = a + (m–1)d

If m^{th} term of A.P is given as n

Then,

a + (m–1)d = n

we get a = n–(m–1)d ......eq 1

a_{n} = a + (n–1)d

If n^{th} term of A.P is given as m

Then,

a + (n–1)d = m

we get a = m–(n–1)d ......eq 2

Equating both eq 1 and eq 2

We get ;

n–(m–1)d = m–(n–1)d

(n–1)d–(m–1)d = m–n

(n–1–(m–1))d = m–n

(n–1–m + 1)d = m–n

(n–m)d = –(n–m)

d = = –1

**Question 1.**

Given a and d for the following A.P., find the following A.P.:

a = 3, d = 2

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

If a = 3, d = 2

Then a_{1} = a + (n–1)d = 3 + (1–1)2 = 3

a_{2} = a + (n–1)d = 3 + (2–1)2 = 3 + 2×1 = 5

a_{3} = a + (n–1)d = 3 + (3–1)2 = 3 + 2×2 = 7

a_{4} = a + (n–1)d = 3 + (4–1)2 = 3 + 2×3 = 9

a_{n} = a + (n–1)d = 3 + (n–1)2 = 2n + 1

∴ The A.P is 3, 5, 7, 9……, 2n + 1

**Question 2.**

Given a and d for the following A.P., find the following A.P.:

a = –3, d = –2

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

If a = –3, d = –2

Then a_{1} = a + (n–1)d = –3 + (1–1)(–2) = –3

a_{2} = a + (n–1)d = –3 + (2–1)(–2) = –3 + 1×(–2) = –5

a_{3} = a + (n–1)d = –3 + (3–1)(–2) = –3 + 2×(–2) = –7

a_{4} = a + (n–1)d = –3 + (4–1)(–2) = –3 + 3×(–2) = –9

a_{n} = a + (n–1)d = –3 + (n–1)(–2) = –1–2n

∴ The A.P is –3, –5, –7, –9……, –1–2n

**Question 3.**

Given a and d for the following A.P., find the following A.P.:

a = 100, d = –7

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

If a = 100, d = –7

Then a_{1} = a + (n–1)d = 100 + (1–1)(–7) = 100

a_{2} = a + (n–1)d = 100 + (2–1)(–7) = 100 + (–7)×1 = 93

a_{3} = a + (n–1)d = 100 + (3–1)(–7) = 100 + (–7)×2 = 86

a_{4} = a + (n–1)d = 100 + (4–1)(–7) = 100 + (–7)×3 = 79

a_{n} = a + (n–1)d = 100 + (n–1)(–7) = 107 – 7n

∴ The A.P is 100, 93, 86, 79……107–7n

**Question 4.**

Given a and d for the following A.P., find the following A.P.:

a = –100, d = 7

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

If a = –100, d = 7

Then a_{1} = a + (n–1)d = –100 + (1–1)7 = –100

a_{2} = a + (n–1)d = –100 + (2–1)7 = –100 + 7×1 = –93

a_{3} = a + (n–1)d = –100 + (3–1)7 = –100 + 7×2 = –86

a_{4} = a + (n–1)d = –100 + (4–1)7 = –100 + 7×3 = –79

a_{n} = a + (n–1)d = –100 + (n–1)7 = 7n–107

∴ The A.P is –100, –93, –86, –79……, 7n–107

**Question 5.**

Given a and d for the following A.P., find the following A.P.:

a = 1000, d = –100

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

If a = 1000, d = –100

Then;

a_{1} = a + (n–1)d = 1000 + (1–1)(–100) = 1000

a_{2} = a + (n–1)d = 1000 + (2–1)(–100) = 1000 + (–100)×1 = 900

a_{3} = a + (n–1)d = 1000 + (3–1)(–100) = 1000 + (–100)×2 = 800

a_{4} = a + (n–1)d = 1000 + (4–1)(–100) = 1000 + (–100)×3 = 700

a_{n} = a + (n–1)d = 1000 + (n–1)(–100) = 1100–100n

∴ The A.P is 1000, 900, 800, 700……, 1100–100n

**Question 6.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

5, –5, 5, –5, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

In the above sequence,

a = 5;

d_{1} = a_{2}–a_{1} = (–5)–5 = –10

d_{2} = a_{3}–a_{2} = 5–(–5) = 10

d_{3} = a_{4}–a_{3} = (–5)–5 = –10

⇒ As in A.P the difference between the 2 terms is always constant

But the difference in sequence is not the same.

∴ The above sequence is not A.P

**Question 7.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

2, 2, 2, 2, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 2;

d_{1} = a_{2}–a_{1} = 2–2 = 0

d_{2} = a_{3}–a_{2} = 2–2 = 0

d_{3} = a_{4}–a_{3} = 2–2 = 0

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence comes to be 0.

∴ The above sequence is not an A.P

**Question 8.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

1, 11, 111, 1111, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

In the above sequence,

a = 1;

d_{1} = a_{2}–a_{1} = 11–1 = 10

d_{2} = a_{3}–a_{2} = 111–11 = 100

d_{3} = a_{4}–a_{3} = 1111–111 = 1000

⇒ As in A.P the difference between the 2 terms is always constant

But the difference in sequence is not the same.

∴ The above sequence is not A.P

**Question 9.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

5, 15, 25, 35, 45, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 5;

d_{1} = a_{2}–a_{1} = 15–5 = 10

d_{2} = a_{3}–a_{2} = 25–15 = 10

d_{3} = a_{4}–a_{3} = 35–25 = 10

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be 10.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 5 + (n–1)10

= 5 + 10n–10

= –5 + 10n

**Question 10.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

17, 22, 27, 32, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 17;

d_{1} = a_{2}–a_{1} = 22–17 = 5

d_{2} = a_{3}–a_{2} = 27–22 = 5

d_{3} = a_{4}–a_{3} = 32–27 = 5

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be 5.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 17 + (n–1)5

= 17 + 5n–5

= 12 + 5n

**Question 11.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

101, 99, 97, 95, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 101;

d_{1} = a_{2}–a_{1} = 99–101 = –2

d_{2} = a_{3}–a_{2} = 97–99 = –2

d_{3} = a_{4}–a_{3} = 95–97 = –2

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be (–2).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 101 + (n–1)(–2)

= 101–2n + 2

= 103–2n

**Question 12.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

201, 198, 195, 192, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 201;

d_{1} = a_{2}–a_{1} = 198–201 = –3

d_{2} = a_{3}–a_{2} = 195–198 = –3

d_{3} = a_{4}–a_{3} = 192–195 = –3

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be (–3).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 201 + (n–1)(–3)

= 201–3n + 3

= 204–3n

**Question 13.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

Natural numbers which are consecutive multiples of 5 in increasing order.

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

The sequence of natural numbers which are consecutive multiples of 5

Is 5, 10, 15, 20 ……

In the above sequence,

a = 5;

d_{1} = a_{2}–a_{1} = 10–5 = 5

d_{2} = a_{3}–a_{2} = 15–10 = 5

d_{3} = a_{4}–a_{3} = 20–15 = 5

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is same and comes to be 5.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 5 + (n–1)5

= 5 + 5n–5

= 5n

**Question 14.**

Determine if the following sequences represent an A.P., assuming that the pattern continues. If it is an A.P., find the nth term:

Natural numbers which are multiples of 3 or 5 in increasing order.

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

The sequence of natural numbers which are multiple of 3 or 5

in increasing order

Is 3, 5, 6, 9, 10 ……

In the above sequence,

a = 3;

d_{1} = a_{2}–a_{1} = 5–3 = 2

d_{2} = a_{3}–a_{2} = 6–5 = 1

d_{3} = a_{4}–a_{3} = 9–6 = 3

⇒ As in A.P the difference between the 2 terms is always constant

The difference in sequence is not same .

∴ The above sequence is not A.P

**Question 15.**

Find the nth term of the following A.P.'s:

2, 7, 12, 17, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 2;

d_{1} = a_{2}–a_{1} = 7–2 = 5

d_{2} = a_{3}–a_{2} = 12–7 = 5

d_{3} = a_{4}–a_{3} = 17–12 = 5

The difference in sequence is same and comes to be 5 .

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 2 + (n–1)(5)

= 2 + 5n–5

= –3 + 5n

**Question 16.**

Find the nth term of the following A.P.'s:

200, 195, 190, 185, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 200;

d_{1} = a_{2}–a_{1} = 195–200 = –5

d_{2} = a_{3}–a_{2} = 190–195 = –5

d_{3} = a_{4}–a_{3} = 185–190 = –5

The difference in sequence is same and comes to be (–5).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 200 + (n–1)(–5)

= 200–5n + 5

= 205–5n

**Question 17.**

Find the nth term of the following A.P.'s:

1000, 900, 800, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 1000;

d_{1} = a_{2}–a_{1} = 900–1000 = –100

d_{2} = a_{3}–a_{2} = 800–900 = –100

The difference in sequence is same and comes to be (–100).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 1000 + (n–1)(–100)

= 1000–100n + 100

= 1100–100n

**Question 18.**

Find the nth term of the following A.P.'s:

50, 100, 150, 200, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 50;

d_{1} = a_{2}–a_{1} = 100–50 = 50

d_{2} = a_{3}–a_{2} = 150–100 = 50

d_{3} = a_{4}–a_{3} = 200–150 = 50

The difference in sequence is same and comes to be 50.

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 50 + (n–1)(50)

= 50 + 50n–50

= 50n

**Question 19.**

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = ;

d_{1} = a_{2}–a_{1} = = 1

d_{2} = a_{3}–a_{2} = = 1

d_{3} = a_{4}–a_{3} = = 1

The difference in sequence is same and comes to be (1).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = + (n–1)(1)

= + n–1

= n–

**Question 20.**

Find the nth term of the following A.P.'s:

1.1, 2.1, 3.1, 4.1, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 1.1;

d_{1} = a_{2}–a_{1} = 2.1–1.1 = 1

d_{2} = a_{3}–a_{2} = 3.1–2.1 = 1

d_{3} = a_{4}–a_{3} = 4.1–3.1 = 1

The difference in sequence is same and comes to be (1).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 1.1 + (n–1)(1)

= 1.1 + n–1

= 0.1 + n

**Question 21.**

Find the nth term of the following A.P.'s:

1.2, 2.3, 3.4, 4.5, ...

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 1.2;

d_{1} = a_{2}–a_{1} = 2.3–1.2 = 1.1

d_{2} = a_{3}–a_{2} = 3.4–2.3 = 1.1

d_{3} = a_{4}–a_{3} = 4.5–3.4 = 1.1

The difference in sequence is same and comes to be (1.1).

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = 1.2 + (n–1)(1.1)

= 1.2 + 1.1n–1.1

= 0.1 + 1.1n

**Question 22.**

Find the nth term of the following A.P.'s:

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = ;

d_{1} = a_{2}–a_{1} = =

d_{2} = a_{3}–a_{2} = =

d_{3} = a_{4}–a_{3} = =

The difference in sequence is same and comes to be ().

∴ The above sequence is A.P

The n^{th} term of A.P is a_{n} = a + (n–1)d

a_{n} = a + (n–1)d = + (n–1)()

=

= 1 + n

**Question 23.**

Find A.P. if Tn, Tm are as given below:

T_{7} = 12, T_{12} = 72

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

a_{7} = a + (7–1)d

a_{7} = a + 6d

If 7^{th} term of A.P is given as 12

Then,

a + 6d = 12

we get a = 12–6d ......eq 1

a_{12} = a + (12–1)d

a_{12} = a + 11d

If 12^{th} term of A.P is given as 72

Then,

a + 11d = 72

we get a = 72–11d ......eq 2

Equating both eq 1 and eq 2

We get ;

12–6d = 72–11d

11d–6d = 72–12

5d = 60

d = = 12

Putting d in eq 1 we get ;

a = 12–6×12

= 12–72 = –60

As a = –60 and d = 12

Then;

a_{1} = a + (n–1)d = –60 + (1–1)(12) = –60

a_{2} = a + (n–1)d = –60 + (2–1)(12) = –60 + (12)×1 = –48

a_{3} = a + (n–1)d = –60 + (3–1)(12) = –60 + (12)×2 = –36

a_{4} = a + (n–1)d = –60 + (4–1)(12) = –60 + (12)×3 = –24

a_{n} = a + (n–1)d = –60 + (n–1)(12) = 12n – 72

∴ The A.P is –60, –48, –36, –24……, 12n – 72

**Question 24.**

Find A.P. if Tn, Tm are as given below:

T_{2} = 1, T_{12} = –9

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

a_{2} = a + (2–1)d

a_{2} = a + d

If 2^{nd} term of A.P is given as 1

Then,

a + d = 1

we get a = 1–d ......eq 1

a_{12} = a + (12–1)d

a_{12} = a + 11d

If 12^{th} term of A.P is given as –9

Then,

a + 11d = –9

we get a = –9–11d ......eq 2

Equating both eq 1 and eq 2

We get ;

1–d = –9–11d

11d–d = –9–1

10d = –10

d = = –1

Putting d in eq 1 we get ;

a = 1–(–1)

= 2

As a = 2 and d = –1

Then;

a_{1} = a + (n–1)d = 2 + (1–1)(–1) = 2

a_{2} = a + (n–1)d = 2 + (2–1)(–1) = 2 + (–1)×1 = 2–1 = 1

a_{3} = a + (n–1)d = 2 + (3–1)(–1) = 2 + (–1)×2 = 2–2 = 0

a_{4} = a + (n–1)d = 2 + (4–1)(–1) = 2 + (–1)×3 = 2–3 = –1

a_{n} = a + (n–1)d = 2 + (n–1)(–1) = 2 + 1–n = 3–n

∴ The A.P is 2, 1, 0, –1……, 3–n

**Question 25.**

In an A.P., T_{3} = 8, T_{10} = T_{6} + 20. Find the A.P.

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

a_{3} = a + (3–1)d

a_{3} = a + 2d

If 3^{rd} term of A.P is given as 8

Then,

a + 2d = 8

we get a = 8–2d ......eq 1

a_{10} = a + (10–1)d

a_{10} = a + 9d

If 10^{th} term of A.P is given as 20 + 6^{th} term

Then,

a_{6} = a + (6–1)d

= a + 5d

a + 9d = 20 + [a + 5d]

we get

a–a + 9d–5d = 20

4d = 20

d = = 5 ......eq 2

Putting d in eq 1 we get ;

a = 8–(2×5)

= 8–10 = –2

As a = –2 and d = 5

Then;

a_{1} = a + (n–1)d = –2 + (1–1)(5) = –2

a_{2} = a + (n–1)d = –2 + (2–1)(5) = –2 + (5)×1 = –2 + 5 = 3

a_{3} = a + (n–1)d = –2 + (3–1)(5) = –2 + (5)×2 = –2 + 10 = 8

a_{4} = a + (n–1)d = –2 + (4–1)(5) = –2 + (5)×3 = –2 + 15 = 13

a_{n} = a + (n–1)d = –2 + (n–1)(5) = –2–5 + 5n = –7 + 5n

∴ The A.P is –2, 3, 8, 13, ……, 5n–7

**Question 26.**

In an A.P. 5th term is 17 and 9th term exceeds 2nd term by 35. Find the A.P.

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

a_{5} = a + (5–1)d

a_{5} = a + 4d

If 5^{th} term of A.P is given as 17

Then,

a + 4d = 17

we get a = 17–4d ......eq 1

a_{9} = a + (9–1)d

a_{9} = a + 8d

If 9^{th} term of A.P is given as 35 + 2^{nd} term

Then,

a_{2} = a + (1)d

= a + d

a + 8d = 35 + [a + d]

we get

a–a + 8d–d = 35

7d = 35

d = = 5 ......eq 2

Putting d in eq 1 we get ;

a = 17–(4×5)

= 17–20 = –3

As a = –3 and d = 5

Then;

a_{1} = a + (n–1)d = –3 + (1–1)(5) = –3

a_{2} = a + (n–1)d = –3 + (2–1)(5) = –3 + (5)×1 = –3 + 5 = 2

a_{3} = a + (n–1)d = –3 + (3–1)(5) = –3 + (5)×2 = –3 + 10 = 7

a_{4} = a + (n–1)d = –3 + (4–1)(5) = –3 + (5)×3 = –3 + 15 = 12

a_{n} = a + (n–1)d = –3 + (n–1)(5) = –3 + 5n–5 = 5n–8

∴ The A.P is –3, 2, 7, 12, ……, 5n–8

**Question 27.**

Can any term of A.P., 12, 17, 22, 27, ... be zero? Why?

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 12;

d_{1} = a_{2}–a_{1} = 17–12 = 5

d_{2} = a_{3}–a_{2} = 22–17 = 5

d_{3} = a_{4}–a_{3} = 27–22 = 5

The difference in sequence is same and comes to be (5).

For any term of A.P to be 0

a_{n} = a + (n–1)d = 0

a_{n} = a + (n–1)d = 12 + (n–1)(5)

= 12 + 5n–5

= 7 + 5n

7 + 5n = 0

5n = –7

n =

∴ The number of terms cannot be negative

Hence, the no term of A.P can be 0

**Question 28.**

Can any term of A.P., 201, 197, 193, ... be 5? Why?

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 201;

d_{1} = a_{2}–a_{1} = 197–201 = –4

d_{2} = a_{3}–a_{2} = 193–197 = –4

The difference in sequence is same and comes to be (–4).

For any term of A.P to be 5

a_{n} = a + (n–1)d = 5

a_{n} = a + (n–1)d = 201 + (n–1)(–4)

5 = 201–4n + 4

5 = 205–4n

–4n = –205 + 5

n = = 50

∴ The 50^{th} term of A.P is 5

**Question 29.**

Which term of A.P., 8, 11, 14, 17, .. is 272?

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 8;

d_{1} = a_{2}–a_{1} = 11–8 = 3

d_{2} = a_{3}–a_{2} = 14–11 = 3

d_{3} = a_{4}–a_{3} = 17–14 = 3

The difference in sequence is same and comes to be (3).

For any term of A.P to be 272

a_{n} = a + (n–1)d = 272

a_{n} = a + (n–1)d = 8 + (n–1)(3)

= 8 + 3n–3

= 5 + 3n

5 + 3n = 272

3n = 272–5 = 267

n = = 89

∴ The 89^{th} term of A.P has value 272

**Question 30.**

Find the 10th term from end for A.P., 3, 6, 9, 12, ... 300.

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 3;

d_{1} = a_{2}–a_{1} = 6–3 = 3

d_{2} = a_{3}–a_{2} = 9–6 = 3

d_{3} = a_{4}–a_{3} = 12–9 = 3

The difference in sequence is same and comes to be (3).

As the last term is 300

a_{n} = a + (n–1)d

300 = 3 + (n–1)3

300 = 3 + 3n–3 = 3n

n = = 100^{th} term

10^{th} term from the end is

100 + 1–10 = 91^{st} term

∵ while counting, last term is also counted

a_{n} = a + (n–1)d

a_{91} = 3 + (91–1)3

= 3 + 90×3

= 273

**Question 31.**

Find the 15th term from end for A.P., 10, 15, 20, 25, 30, ..., 1000.

**Answer:**

Formula Used.

d_{n} = a_{n + 1} – a_{n}

a_{n} = a + (n–1)d

In the above sequence,

a = 10;

d_{1} = a_{2}–a_{1} = 15–10 = 5

d_{2} = a_{3}–a_{2} = 20–15 = 5

d_{3} = a_{4}–a_{3} = 25–20 = 5

The difference in sequence is same and comes to be (5).

As the last term is 1000

a_{n} = a + (n–1)d

1000 = 10 + (n–1)5

1000 = 10 + 5n–5 = 5 + 5n

5n = 995

n = = 199^{th} term

15 term from the end will be

199 + 1–15 = 185^{th} term

a_{n} = a + (n–1)d

a_{185} = 10 + (185–1)5

= 10 + 184×5

= 10 + 920

= 930

**Question 32.**

If in an A.P., T_{7} = 18, T_{18} = 7, find T_{101}.

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

a_{7} = a + (7–1)d

a_{7} = a + 6d

If 7^{th} term of A.P is given as 18

Then,

a + 6d = 18

we get a = 18–6d ......eq 1

a_{18} = a + (18–1)d

a_{18} = a + 17d

If 18^{th} term of A.P is given as 7

Then,

a + 17d = 7

we get a = 7–17d ......eq 2

Equating both eq 1 and eq 2

We get ;

18–6d = 7–17d

17d–6d = 7–18

11d = –11

d = = –1

Putting d in eq 1 we get ;

a = 18–6×(–1)

= 18 + 6 = 24

For 101^{st} term

a_{n} = a + (n–1)d

a_{101} = 24 + (101–1)(–1)

= 24–100

= –76

**Question 33.**

If in an A.P., T_{m} = n, T_{n} = m, prove d = –1.

**Answer:**

Formula Used.

a_{n} = a + (n–1)d

a_{m} = a + (m–1)d

If m^{th} term of A.P is given as n

Then,

a + (m–1)d = n

we get a = n–(m–1)d ......eq 1

a_{n} = a + (n–1)d

If n^{th} term of A.P is given as m

Then,

a + (n–1)d = m

we get a = m–(n–1)d ......eq 2

Equating both eq 1 and eq 2

We get ;

n–(m–1)d = m–(n–1)d

(n–1)d–(m–1)d = m–n

(n–1–(m–1))d = m–n

(n–1–m + 1)d = m–n

(n–m)d = –(n–m)

d = = –1

###### Exercise 5.2

**Question 1.**Find the sum of the first n terms of the following A.P. as asked for:

2, 6, 10, 14, ... up to 20 terms

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = 2,

d = a_{2}–a_{1}

= 6–2 = 4;

n = 20

S_{n} = [2a + (n–1)d]

= [2×2 + (20–1)4]

= 10[4 + 19×4]

= 10[4 + 76]

= 800

**Question 2.**Find the sum of the first n terms of the following A.P. as asked for:

5, 7, 9, 11, ... upto 30 terms

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = 5,

d = a_{2}–a_{1}

= 7–5 = 2;

n = 30

S_{n} = [2a + (n–1)d]

= [2×5 + (30–1)2]

= 15[10 + 29×2]

= 15[10 + 58]

= 1020

**Question 3.**Find the sum of the first n terms of the following A.P. as asked for:

—10, —12, —14, —16, ... up to 15 terms

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = –10,

d = a_{2}–a_{1}

= –12–(–10) = (–2);

n = 15

S_{n} = [2a + (n–1)d]

= [2×(–10) + (15–1)(–2)]

= [–20 + 14×(–2)]

= [–20–28]

= 15×(–24)

= –360

**Question 4.**Find the sum of the first n terms of the following A.P. as asked for:

1, 1.5, 2, 2.5, 3, ... upto 16 terms

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = 1,

d = a_{2}–a_{1}

= 1.5–1 = 0.5;

n = 16

S_{n} = [2a + (n–1)d]

= [2×1 + (16–1)0.5]

= 8[2 + 15×0.5]

= 8[2 + 7.5]

= 76

**Question 5.**Find the sum of the first n terms of the following A.P. as asked for:

up to 18 terms

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = ,

d = a_{2}–a_{1}

= = 1;

n = 18

S_{n} = [2a + (n–1)d]

= [2× + (18–1)1]

= 9[ + 17]

= 9[]

= 159

**Question 6.**Find the sums indicated below:

3 + 6 + 9 + ... + 300

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 3, 6, 9, ……, 300

In the following A.P

⇒ a = 3,

⇒ d = 6–3 = 3

As the last term is 300

a_{n} = a + (n–1)d

300 = 3 + (n–1)3

300 = 3 + 3n–3

3n = 300

⇒ n = = 100

S_{n} = [2a + (n–1)d]

= [2×3 + (100–1)3]

= 50[6 + 99×3]

= 50[6 + 297]

= 15150

**Question 7.**Find the sums indicated below:

5 + 10 + 15 + …… + 100

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 5, 10, 15, ……, 100

In the following A.P

⇒ a = 5,

⇒ d = 10–5 = 5

As the last term is 100

a_{n} = a + (n–1)d

100 = 5 + (n–1)5

100 = 5 + 5n–5

5n = 100

⇒ n = = 20

S_{n} = [2a + (n–1)d]

= [2×5 + (20–1)5]

= 10[10 + 19×5]

= 10[10 + 95]

= 1050

**Question 8.**Find the sums indicated below:

7 + 12 + 17 + 22 + …… + 102

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 7, 12, 17, 22, ……, 102

In the following A.P

⇒ a = 7,

⇒ d = 12–7 = 5

As the last term is 102

a_{n} = a + (n–1)d

102 = 7 + (n–1)5

102 = 7 + 5n–5

5n = 102–2

⇒ n = = 20

S_{n} = [2a + (n–1)d]

= [2×7 + (20–1)5]

= 10[14 + 19×5]

= 10[14 + 95]

= 1090

**Question 9.**Find the sums indicated below:

(–100) + (–92) + (–84) + …… + 92

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is –100, –92, –84, ……, 92

In the following A.P

⇒ a = –100,

⇒ d = –92–(–100) = –92 + 100 = 8

As the last term is 92

a_{n} = a + (n–1)d

92 = –100 + (n–1)8

92 = –100 + 8n–8

8n = 92 + 108

⇒ n = = 25

S_{n} = [2a + (n–1)d]

= [2×(–100) + (25–1)8]

= [–200 + 24×8]

= [–200 + 192]

= –100

**Question 10.**Find the sums indicated below:

25 + 21 + 17 + 13 + …… + (–51)

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 25, 21, 17, 13, ……, (–51)

In the following A.P

⇒ a = 25,

⇒ d = 21–25 = –4

As the last term is –51

a_{n} = a + (n–1)d

–51 = 25 + (n–1)(–4)

–51 = 25–4n + 4

4n = 80

⇒ n = = 20

S_{n} = [2a + (n–1)d]

= [2×25 + (20–1)(–4)]

= 10[50 + 19×(–4)]

= 10[50–76]

= –260

**Question 11.**For a given A.P. with

a = 1, d = 2, find S_{10}.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

⇒ a = 1,

⇒ d = 2

⇒ n = 10

S_{n} = [2a + (n–1)d]

= [2×1 + (10–1)2]

= 5[2 + 9×2]

= 5[2 + 18]

= 100

**Question 12.**For a given A.P. with

a = 2, d = 3, find S_{30}.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

⇒ a = 2,

⇒ d = 3

⇒ n = 30

S_{n} = [2a + (n–1)d]

= [2×2 + (30–1)3]

= 15[4 + 29×3]

= 15[4 + 87]

= 1365

**Question 13.**For a given A.P. with

S_{3} = 9, S_{7} = 49, find S_{n} and S_{10}.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

⇒ S_{3} = 9,

S_{3} = [2a + (3–1)d]

9 = [2a + 2d]

9 = [2(a + d)]

= [a + d]

3 = [a + d]

⇒ a = 3–d ……eq 1

⇒ S_{7} = 49

S_{7} = [2a + (7–1)d]

49 = [2a + 6d]

49 = [2(a + 3d)]

= a + 3d

7 = a + 3d

⇒ a = 7–3d ......eq 2

By equating eq 1 and eq 2

3–d = 7–3d

3d – d = 7 – 3

2d = 4

d = = 2

Putting value of d in eq 1

a = 3–d = 3–2

= 1

S_{n} = [2a + (n–1)d]

= [2×1 + (n–1)2]

= [2 + 2n–2]

= ×2n

= n^{2}

∴ S_{n} = n^{2}

⇒ S_{10} = (10)^{2}

= 100

**Question 14.**For a given A.P. with

T_{10} = 41, S10 = 320, find T_{n}, S_{n}.

**Answer:**Formula used.

S_{n} = [a + a_{n}]

T_{n} = a_{n} = a + (n–1)d

S_{n} = [2a + (n–1)d]

⇒ S_{10} = 320

⇒ T_{10} = a_{10} = 41

S_{10} = [a + 41]

320 = [a + 41]

= a + 41

64 = a + 41

a = 64–41

⇒ a = 23

a_{10} = a + (n–1)d

41 = 23 + (10–1)d

41–23 = 9d

9d = 18

⇒ d = = 2

T_{n} = a_{n} = a + (n–1)d

T_{n} = 23 + (n–1)2

23–2 + 2n

21 + 2n

S_{n} = [2a + (n–1)d]

S_{n} = [2×23 + (n–1)2]

= [46 + 2n–2]

= [44 + 2n]

= n[22 + n]

= 22n + n^{2}

**Question 15.**For a given A.P. with

S_{10} = 50, a = 0.5, find d.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

⇒ S_{10} = 50

⇒ n = 10

⇒ a = 0.5S_{10} = [2×0.5 + (10–1)d]

50 = 5[1 + 9d]

= 1 + 9d

9d = 10–1

d = = 1

**Question 16.**For a given A.P. with

S_{20} = 100, d = —2, find a.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

⇒ S_{20} = 100

⇒ n = 20

⇒ d = –2

S_{20} = [2a + (20–1)(–2)]

100 = 10[2a–38]

100 = 20[a–19]

= a–19

5 = a–19

a = 19 + 5

a = 24

**Question 17.**How many terms of A.P., 2, 7, 12, 17, ... add up to 990?

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

⇒ S_{20} = 990

⇒ a = 2

⇒ d = a_{n + 1}–a_{n}

= 7 – 2 = 5

S_{n} = [2×2 + (n–1)5]

990 = [4 + 5n–5]

990 = [5n–1]

990×2 = 5n^{2}–n

5n^{2}–n–1980 = 0

n =

∴ n can be 20 or –19.8

Since, the number of terms cannot be negative

∴ Sum of 20 terms gives the sum of 990

**Question 18.**The first term of finite A.P. is 5, the last term is 45 and the sum is 500. Find the number of terms.

**Answer:**Formula used.

S_{n} = [a + l]

⇒ S_{n} = 500

⇒ a = 5

⇒ l = 45

S_{n} = [a + l]

500 = [5 + 45]

500×2 = n×50

n = = 20

∴ Number of terms to get sum 500 is 20

**Question 19.**If the first term and the last term of a finite A.P. are 5 and 95 respectively and d = 5, find n and S_{n}.

**Answer:**Formula used.

a_{n} = a + (n–1)d

S_{n} = [2a + (n–1)d]

⇒ d = 5

⇒ a = 5

⇒ l = 95

a_{n} = a + (n–1)d

95 = 5 + (n–1)5

95 = 5–5 + 5n

n = = 19

S_{n} = [a + l]

= [5 + 95]

= 19×50

= 950

∴ Number of terms is 19 and Sum of A.P is 950

**Question 20.**The sum of first n terms of an A.P. is 5n — 2n^{2}. Find the A.P. i.e. a and d.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

S_{n} = [2a + (n–1)d]

5n – 2n^{2} = [2a + (n–1)d]

2×n[5 – 2n] = n[2a + (n–1)d]

10 – 4n = (2a–d) + nd

Then

Real part should be equal to Real

And imaginary part should be equal to imaginary

2a – d = 10 ......eq 1

nd = –4n

⇒ d = –4

Putting value of d in eq 1

2a – d = 10

2a – (–4) = 10

2a + 4 = 10

2a = 10 – 4 = 6

⇒ a = = 3

**Question 21.**Find the sum of all three–digit numbers divisible by 3.

**Answer:**Formula used.

S_{n} = [a + l]

a_{n} = a + (n–1)d

⇒ The 1^{st} 3 digit number divisible by 3 is 102

For the last 3 digit number divisible by 3

Divide 999 by 3

It gets completely divisible by 3

⇒ The last 3 digit number divisible by 3 is 999

a_{n} = a + (n–1)d

999 = 102 + (n–1)3

999 = 102–3 + 3n

3n = 999 – 99

n = = 300

S_{n} = [a + l]

= [102 + 999]

= 150×1101

= 165150

**Question 22.**Find the sum of all odd numbers from 5 to 205.

**Answer:**Formula used.

S_{n} = [a + l]

a_{n} = a + (n–1)d

⇒ The 1^{st} odd number is 5

⇒ The last odd number is 205

⇒ Difference between every odd number is 2

a_{n} = a + (n–1)d

205 = 5 + (n–1)2

205 = 5–2 + 2n

2n = 205 – 3

n = = 101

S_{n} = [a + l]

= [5 + 205]

= 101×105

= 10605

**Question 23.**Which term of A.P. 121, 117, 113, ... is its first negative term? If it is the nth term, find S_{n}.

**Answer:**Formula used.

S_{n} = [2a + (n–1)d]

a_{n} = a + (n–1)d

d = a_{n + 1} – a_{n}

Let the 1^{st} negative term be X

a = 121

d = 117 – 121

= –4

If the difference is –4 then

Then X can be (–1), (–2), (–3), (–4)

a_{n} = a + (n–1)d

X = 121 + (n–1)(–4)

X = 125–4n

4n = 125 – X

n =

For the number of terms to be an integer (125–X) will be multiple of 4

When we put X = –1

n will be

which is not an integer

When we put X = –2

n will be

which is not an integer

When we put X = –3

n will be = 32

which is an integer

Hence –3 is the 1^{st} negative term.

S_{n} = [2a + (n–1)d]

= [2×121 + (32–1)(–4)]

= 16[242 – 124]

= 16×118

= 1888

**Question 1.**

Find the sum of the first n terms of the following A.P. as asked for:

2, 6, 10, 14, ... up to 20 terms

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = 2,

d = a_{2}–a_{1}

= 6–2 = 4;

n = 20

S_{n} = [2a + (n–1)d]

= [2×2 + (20–1)4]

= 10[4 + 19×4]

= 10[4 + 76]

= 800

**Question 2.**

Find the sum of the first n terms of the following A.P. as asked for:

5, 7, 9, 11, ... upto 30 terms

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = 5,

d = a_{2}–a_{1}

= 7–5 = 2;

n = 30

S_{n} = [2a + (n–1)d]

= [2×5 + (30–1)2]

= 15[10 + 29×2]

= 15[10 + 58]

= 1020

**Question 3.**

Find the sum of the first n terms of the following A.P. as asked for:

—10, —12, —14, —16, ... up to 15 terms

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = –10,

d = a_{2}–a_{1}

= –12–(–10) = (–2);

n = 15

S_{n} = [2a + (n–1)d]

= [2×(–10) + (15–1)(–2)]

= [–20 + 14×(–2)]

= [–20–28]

= 15×(–24)

= –360

**Question 4.**

Find the sum of the first n terms of the following A.P. as asked for:

1, 1.5, 2, 2.5, 3, ... upto 16 terms

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = 1,

d = a_{2}–a_{1}

= 1.5–1 = 0.5;

n = 16

S_{n} = [2a + (n–1)d]

= [2×1 + (16–1)0.5]

= 8[2 + 15×0.5]

= 8[2 + 7.5]

= 76

**Question 5.**

Find the sum of the first n terms of the following A.P. as asked for:

up to 18 terms

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

a = ,

d = a_{2}–a_{1}

= = 1;

n = 18

S_{n} = [2a + (n–1)d]

= [2× + (18–1)1]

= 9[ + 17]

= 9[]

= 159

**Question 6.**

Find the sums indicated below:

3 + 6 + 9 + ... + 300

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 3, 6, 9, ……, 300

In the following A.P

⇒ a = 3,

⇒ d = 6–3 = 3

As the last term is 300

a_{n} = a + (n–1)d

300 = 3 + (n–1)3

300 = 3 + 3n–3

3n = 300

⇒ n = = 100

S_{n} = [2a + (n–1)d]

= [2×3 + (100–1)3]

= 50[6 + 99×3]

= 50[6 + 297]

= 15150

**Question 7.**

Find the sums indicated below:

5 + 10 + 15 + …… + 100

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 5, 10, 15, ……, 100

In the following A.P

⇒ a = 5,

⇒ d = 10–5 = 5

As the last term is 100

a_{n} = a + (n–1)d

100 = 5 + (n–1)5

100 = 5 + 5n–5

5n = 100

⇒ n = = 20

S_{n} = [2a + (n–1)d]

= [2×5 + (20–1)5]

= 10[10 + 19×5]

= 10[10 + 95]

= 1050

**Question 8.**

Find the sums indicated below:

7 + 12 + 17 + 22 + …… + 102

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 7, 12, 17, 22, ……, 102

In the following A.P

⇒ a = 7,

⇒ d = 12–7 = 5

As the last term is 102

a_{n} = a + (n–1)d

102 = 7 + (n–1)5

102 = 7 + 5n–5

5n = 102–2

⇒ n = = 20

S_{n} = [2a + (n–1)d]

= [2×7 + (20–1)5]

= 10[14 + 19×5]

= 10[14 + 95]

= 1090

**Question 9.**

Find the sums indicated below:

(–100) + (–92) + (–84) + …… + 92

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is –100, –92, –84, ……, 92

In the following A.P

⇒ a = –100,

⇒ d = –92–(–100) = –92 + 100 = 8

As the last term is 92

a_{n} = a + (n–1)d

92 = –100 + (n–1)8

92 = –100 + 8n–8

8n = 92 + 108

⇒ n = = 25

S_{n} = [2a + (n–1)d]

= [2×(–100) + (25–1)8]

= [–200 + 24×8]

= [–200 + 192]

= –100

**Question 10.**

Find the sums indicated below:

25 + 21 + 17 + 13 + …… + (–51)

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the sum above A.P follows

Is 25, 21, 17, 13, ……, (–51)

In the following A.P

⇒ a = 25,

⇒ d = 21–25 = –4

As the last term is –51

a_{n} = a + (n–1)d

–51 = 25 + (n–1)(–4)

–51 = 25–4n + 4

4n = 80

⇒ n = = 20

S_{n} = [2a + (n–1)d]

= [2×25 + (20–1)(–4)]

= 10[50 + 19×(–4)]

= 10[50–76]

= –260

**Question 11.**

For a given A.P. with

a = 1, d = 2, find S_{10}.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

⇒ a = 1,

⇒ d = 2

⇒ n = 10

S_{n} = [2a + (n–1)d]

= [2×1 + (10–1)2]

= 5[2 + 9×2]

= 5[2 + 18]

= 100

**Question 12.**

For a given A.P. with

a = 2, d = 3, find S_{30}.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

⇒ a = 2,

⇒ d = 3

⇒ n = 30

S_{n} = [2a + (n–1)d]

= [2×2 + (30–1)3]

= 15[4 + 29×3]

= 15[4 + 87]

= 1365

**Question 13.**

For a given A.P. with

S_{3} = 9, S_{7} = 49, find S_{n} and S_{10}.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

In the following A.P

⇒ S_{3} = 9,

S_{3} = [2a + (3–1)d]

9 = [2a + 2d]

9 = [2(a + d)]

= [a + d]

3 = [a + d]

⇒ a = 3–d ……eq 1

⇒ S_{7} = 49

S_{7} = [2a + (7–1)d]

49 = [2a + 6d]

49 = [2(a + 3d)]

= a + 3d

7 = a + 3d

⇒ a = 7–3d ......eq 2

By equating eq 1 and eq 2

3–d = 7–3d

3d – d = 7 – 3

2d = 4

d = = 2

Putting value of d in eq 1

a = 3–d = 3–2

= 1

S_{n} = [2a + (n–1)d]

= [2×1 + (n–1)2]

= [2 + 2n–2]

= ×2n

= n^{2}

∴ S_{n} = n^{2}

⇒ S_{10} = (10)^{2}

= 100

**Question 14.**

For a given A.P. with

T_{10} = 41, S10 = 320, find T_{n}, S_{n}.

**Answer:**

Formula used.

S_{n} = [a + a_{n}]

T_{n} = a_{n} = a + (n–1)d

S_{n} = [2a + (n–1)d]

⇒ S_{10} = 320

⇒ T_{10} = a_{10} = 41

S_{10} = [a + 41]

320 = [a + 41]

= a + 41

64 = a + 41

a = 64–41

⇒ a = 23

a_{10} = a + (n–1)d

41 = 23 + (10–1)d

41–23 = 9d

9d = 18

⇒ d = = 2

T_{n} = a_{n} = a + (n–1)d

T_{n} = 23 + (n–1)2

23–2 + 2n

21 + 2n

S_{n} = [2a + (n–1)d]

S_{n} = [2×23 + (n–1)2]

= [46 + 2n–2]

= [44 + 2n]

= n[22 + n]

= 22n + n^{2}

**Question 15.**

For a given A.P. with

S_{10} = 50, a = 0.5, find d.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

⇒ S_{10} = 50

⇒ n = 10

⇒ a = 0.5S_{10} = [2×0.5 + (10–1)d]

50 = 5[1 + 9d]

= 1 + 9d

9d = 10–1

d = = 1

**Question 16.**

For a given A.P. with

S_{20} = 100, d = —2, find a.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

⇒ S_{20} = 100

⇒ n = 20

⇒ d = –2

S_{20} = [2a + (20–1)(–2)]

100 = 10[2a–38]

100 = 20[a–19]

= a–19

5 = a–19

a = 19 + 5

a = 24

**Question 17.**

How many terms of A.P., 2, 7, 12, 17, ... add up to 990?

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

d = a_{n + 1}–a_{n}

⇒ S_{20} = 990

⇒ a = 2

⇒ d = a_{n + 1}–a_{n}

= 7 – 2 = 5

S_{n} = [2×2 + (n–1)5]

990 = [4 + 5n–5]

990 = [5n–1]

990×2 = 5n^{2}–n

5n^{2}–n–1980 = 0

n =

∴ n can be 20 or –19.8

Since, the number of terms cannot be negative

∴ Sum of 20 terms gives the sum of 990

**Question 18.**

The first term of finite A.P. is 5, the last term is 45 and the sum is 500. Find the number of terms.

**Answer:**

Formula used.

S_{n} = [a + l]

⇒ S_{n} = 500

⇒ a = 5

⇒ l = 45

S_{n} = [a + l]

500 = [5 + 45]

500×2 = n×50

n = = 20

∴ Number of terms to get sum 500 is 20

**Question 19.**

If the first term and the last term of a finite A.P. are 5 and 95 respectively and d = 5, find n and S_{n}.

**Answer:**

Formula used.

a_{n} = a + (n–1)d

S_{n} = [2a + (n–1)d]

⇒ d = 5

⇒ a = 5

⇒ l = 95

a_{n} = a + (n–1)d

95 = 5 + (n–1)5

95 = 5–5 + 5n

n = = 19

S_{n} = [a + l]

= [5 + 95]

= 19×50

= 950

∴ Number of terms is 19 and Sum of A.P is 950

**Question 20.**

The sum of first n terms of an A.P. is 5n — 2n^{2}. Find the A.P. i.e. a and d.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

S_{n} = [2a + (n–1)d]

5n – 2n^{2} = [2a + (n–1)d]

2×n[5 – 2n] = n[2a + (n–1)d]

10 – 4n = (2a–d) + nd

Then

Real part should be equal to Real

And imaginary part should be equal to imaginary

2a – d = 10 ......eq 1

nd = –4n

⇒ d = –4

Putting value of d in eq 1

2a – d = 10

2a – (–4) = 10

2a + 4 = 10

2a = 10 – 4 = 6

⇒ a = = 3

**Question 21.**

Find the sum of all three–digit numbers divisible by 3.

**Answer:**

Formula used.

S_{n} = [a + l]

a_{n} = a + (n–1)d

⇒ The 1^{st} 3 digit number divisible by 3 is 102

For the last 3 digit number divisible by 3

Divide 999 by 3

It gets completely divisible by 3

⇒ The last 3 digit number divisible by 3 is 999

a_{n} = a + (n–1)d

999 = 102 + (n–1)3

999 = 102–3 + 3n

3n = 999 – 99

n = = 300

S_{n} = [a + l]

= [102 + 999]

= 150×1101

= 165150

**Question 22.**

Find the sum of all odd numbers from 5 to 205.

**Answer:**

Formula used.

S_{n} = [a + l]

a_{n} = a + (n–1)d

⇒ The 1^{st} odd number is 5

⇒ The last odd number is 205

⇒ Difference between every odd number is 2

a_{n} = a + (n–1)d

205 = 5 + (n–1)2

205 = 5–2 + 2n

2n = 205 – 3

n = = 101

S_{n} = [a + l]

= [5 + 205]

= 101×105

= 10605

**Question 23.**

Which term of A.P. 121, 117, 113, ... is its first negative term? If it is the nth term, find S_{n}.

**Answer:**

Formula used.

S_{n} = [2a + (n–1)d]

a_{n} = a + (n–1)d

d = a_{n + 1} – a_{n}

Let the 1^{st} negative term be X

a = 121

d = 117 – 121

= –4

If the difference is –4 then

Then X can be (–1), (–2), (–3), (–4)

a_{n} = a + (n–1)d

X = 121 + (n–1)(–4)

X = 125–4n

4n = 125 – X

n =

For the number of terms to be an integer (125–X) will be multiple of 4

When we put X = –1

n will be

which is not an integer

When we put X = –2

n will be

which is not an integer

When we put X = –3

n will be = 32

which is an integer

Hence –3 is the 1^{st} negative term.

S_{n} = [2a + (n–1)d]

= [2×121 + (32–1)(–4)]

= 16[242 – 124]

= 16×118

= 1888

###### Exercise 5

**Question 1.**If T_{n} = 6n + 5, find S_{n}

**Answer:**From the question we know that, T_{n} = 6n + 5 ……. (1)

And we know that T_{n} = a + (n – 1)d

So we have,

⇒ a + (n – 1)d = 6n + 5

⇒ a + (n – 1)d = 6n + 11 – 6

⇒ a + (n – 1)d = 11 + (6n – 6)

⇒ a + (n – 1)d = 11 + 6(n – 1)

On comparing both the sides, we have:

⇒ a = 11 and d = 6

So now, S_{n} = × (2a + (n – 1)d)

⇒ S_{n} = × (2(11) + (n – 1)(6))

⇒ S_{n} = × (22 + 6n – 6)

⇒ S_{n} = × (16 + 6n)

⇒ S_{n} = n × (8 + 3n)

⇒ S_{n} = 3n^{2} + 8n

∴ Sum of n terms of the given A.P is S_{n} = 3n^{2} + 8n

**Question 2.**If S_{n} = n^{2} + 2n, find T_{n}

**Answer:**From the question we know that, S_{n} = n^{2} + 2n ……. (1)

We know that, S_{n} = × (2a + (n – 1)d)

= na +

= na + (n^{2} – n) ×

S_{n} = na + n^{2} × – n × ……. (2)

As we know that, (1) and (2) are both sum of the same arithmetic progression

So we can equate them to each other.

So, we have,

⇒ n^{2} + 2n = na + n^{2} × – n ×

⇒ n^{2} + 2n = n^{2} × + na – n ×

Now we will equate the coefficients of “n^{2}” and “n” on both the sides of the equal to sign.

So, we get,

For coefficients of n^{2} :

⇒ 1 =

⇒ d=2

Now, For coefficients of n:

⇒ 2 = a –

From above we have that d=2,

So we can say that,

⇒ 2 = a –

⇒ 2 = a – 1

⇒ a=3

So, now, we know that, T_{n} = a + (n – 1)d

Now we put values of a and n in the above equation,

We get,

⇒ T_{n} = 3 + (n – 1)2

⇒ T_{n} = 3 + 2n – 2

⇒ T_{n} = 2n + 1

∴ for the given value of S_{n} = n^{2} + 2n, we have T_{n} = 2n + 1.

**Question 3.**If the sum of the first n terms of A, P.

30, 27, 24, 21 … is 120, find the number of terms and the last term.

**Answer:**We can see that, A.P. is 30, 27, 24, 21 …

T_{1} = 30 = a

T_{2} = 27

So, we have d = T_{2} – T_{1}

d = 27 – 30

d= – 3

Now, we have sum of the A.P. S_{n} = × (2a + (n – 1)d)

And it is given that, S_{n} = 120

So, we have,

⇒ 120 = × (2a + (n – 1)d)

Now we will put the values of a and d in the above equation

⇒ 120 = × (2(30) + (n – 1)(– 3))

⇒ 120 = × (60 + 3 – 3n)

120 × 2 = n × (63 – 3n)

⇒ 240 = 63n – 3n^{2}

⇒ 3n^{2} – 63n + 240 = 0

⇒ n^{2} – 21n + 80 = 0

⇒ n^{2} – 16n – 5n + 80 = 0

⇒ n(n – 16) – 5(n – 16) = 0

⇒ (n – 16)(n – 5) = 0

⇒ n= 16 or 5

Now, when n=16, the last term = T_{16}

So we have, T_{16} = a + (16 – 1)d ……. (∵ T_{n} = a + (n – 1)d )

⇒ T_{16} = a + 15d

Now we put the values of a and d in the above equation

⇒ T_{16}= 30 + 15(– 3)

⇒ T_{16} = 30 – 45

⇒ T_{16} = – 15

Now, when n=5, the last term = T_{5}

So we have, T5 = a + (5 – 1)d ……. (∵ Tn = a + (n – 1)d )

⇒ T_{5} = a + 4d

Now we put the values of a and d in the above equation

⇒ T_{5}= 30 + 4(– 3)

⇒ T_{5} = 30 – 12

⇒ T_{5} = 18

∴ The no. Of term in the above A.P. can be n = 5 or 16

And the last terms of the respective A.Ps are:

For n = 5, last term T5 = 18

For n = 16, last term T16 = – 15

**Question 4.**Which term of A.P., 100, 97, 94, 91, will be its first —ve term?

**Answer:**We have the A.P as 100, 97, 94, 91 …

So, a = 100

⇒ d= 97 – 100 = – 3

Now recall that, nth term of an AP is T_{n} = a + (n – 1)d

⇒ T_{n} = 100 + (n – 1)(– 3)

= 100 – 3n + 3

⇒ T_{n} = 103 – 3n

Since we need negative term, T_{n} < 0

So, 103 – 3n < 0

⇒ 103 < 3n

⇒ < n

⇒ 34.333 < n

⇒ n>34

i.e. N = 35

∴ n=35 for the first negative term in the given A.P.

⇒ T_{35} = a + (35 – 1)d

= 100 + 34 × – 3

= 100 – 102

= – 2

∴ the first negative term of the given AP is – 2.

**Question 5.**Find the sum of all 3 digit natural multiples of 6.

**Answer:**Now the first 3 digit multiple of 6 is 102.

That means, a = 102

And all the nos. Are multiples of 6 which means that they have a difference of 6 in between them so, common difference d = 6

Now, the last 3 digit multiple of 6 is 996.

Now, we know that, T_{n} = a + (n – 1)d

So, 996 = a + (n – 1)d

⇒ 996 = 102 + (n – 1)6

⇒ 6(n – 1) = 996 – 102

⇒ 6(n – 1) = 894

⇒ n – 1 =

⇒ n – 1 = 149

⇒ n = 150

So, we can say that there are 150, 3 digit multiples of 6

Now sum of these nos. S_{150} = × (2a + (n – 1)d)

⇒ S_{150} = × (2(102) + (150 – 1)6)

= 75 × (204 + 149 × 6)

= 75 × (1098)

⇒ S_{150} = 82350

∴ Sum of all 3 digit natural multiples of 6 is 82350.

**Question 6.**The ratio of the sum of m terms to sum of n terms of an A.P. is. Find the ratio of its mth term to its nth term.

**Answer:**As we know that sum of n terms of an A.P is:

⇒ S_{n}= × (2a + (n – 1)d)

So, sum of m terms of an A.P is:

⇒ S_{m} = × (2a + (m – 1)d)

From the question we know that,

⇒ =

⇒ =

⇒ =

⇒ =

⇒ m × (2a + (n – 1)d) = n × (2a + (m – 1)d)

⇒ 2ma + m(n – 1)d = 2na + n(m – 1)d

⇒ 2ma + md(n – 1) = 2na + nd(m – 1)

⇒ 2ma + mnd – md = 2na + mnd – nd

⇒ 2ma – md = 2na – nd

⇒ 2ma – 2na = md – nd

⇒ 2a(m – n) = d(m – n)

⇒ 2a = d

Now, m^{th} term of the given A.P is T_{m} = a + (m – 1)d

Now, n^{th} term of the given A.P is T_{n} = a + (n – 1)d

Now ratio between these 2 terms is:

⇒ =

Now we have, 2a = d

So we get,

⇒ =

⇒ =

⇒ =

⇒ =

⇒ =

∴ the ratio of m^{th} term of the given A.P to its n^{th} Term is:

T_{m} : T_{n} = 2m – 1 : 2n – 1

**Question 7.**Sum to first l, m, n terms of A.P. are p, q, r. Prove that

**Answer:**Let the first term of AP be a and common difference be d.

Sum of the first p terms is:

S_{l} = × (2a + (l – 1)d) = p ………(1)

Sum of the first m terms is:

⇒ S_{m} = × (2a + (m – 1)d) = q ……… (2)

Sum of the first n terms is:

⇒ S_{n} = × (2a + (n – 1)d) = r ……… (3)

Now, (1) × + (2) × + (3) ×

We get, p × + q × + r ×

= 0 + [(m – n)l + (n – l)m + (l – m)n –m + n – n + l – l + m]

= [lm – ln + mn – lm + ln – mn + 0]

= × 0 = 0

∴ it is proved that, p × + q × + r × = 0

**Question 8.**The ratio of sum to n terms of two A.P. is for every n ∈ N. Find the ratio of their 7^{th} terms and m^{th} terms.

**Answer:**Let the sum of n terms of the first A.P be:

⇒ S_{n} = × (2a + (n – 1)d) …………… (1)

Let the sum of n terms of the second A.P be:

⇒ S’_{n} = × (2a’ + (n – 1)d’) …………… (2)

Now according to the question:

⇒ =

Let’s consider the ratio these two AP’s m^{th} terms as:

T_{m} : T’_{m}

Now, recall that, nth term of an AP is T_{n} = a + (n – 1)d

⇒ T_{m} = a + (m – 1)d

⇒ T’^{m} = a’ + (m – 1)d’

Hence the ratio of these two AP’s m^{th} terms become:

⇒ =

On multiplying by 2, we get,

⇒ =

⇒ =

⇒ =

⇒ =

⇒ =

⇒ =

Now from the above formula of the ratio of mth terms of 2 Aps, we can find the ratio of 7^{th} terms of both Aps

So we have, =

⇒ =

⇒ =

∴ the ratio of m^{th} terms of the given 2 Aps is, 16m – 7 : 14m – 4

∴ the ratio of 7^{th} terms of the given 2 Aps is, 105 : 94

**Question 9.**Three numbers in A.P. have the sum 18 and the sum of their squares is 180. Find the numbers in the increasing order.

**Answer:**We know that the sum of 3 numbers in the AP is 18.

Let’s say that those 3 numbers are: a – d, a, a + d

So we can say that, a – d + a + a + d = 18

⇒ 3a = 18

⇒ a= 6

Now, we have that the sum of squares of these 3 numbers is 180.

So we can say that, (a – d) ^{2} + a^{2} + (a + d) ^{2} = 180

⇒ a^{2} – 2ad + d^{2} + a^{2} + a^{2} + 2ad + d^{2} = 180

⇒ 3a^{2} + 2d^{2} = 180

We know that a = 6,

So, 3(6)^{2} + 2d^{2} = 180

⇒ 3(36) + 2d^{2} = 180

⇒ 108 + 2d^{2} = 180

⇒ 2d^{2} = 180 – 108

⇒ 2d^{2} = 72

⇒ d^{2} = 36

⇒ d = 6

Now, our 3 numbers a – d, a, a + d are 6 – 6, 6, 6 + 6 = 0, 6, 12 respectively.

∴ The 3 numbers of the A.P in the increasing order are : 0, 6, 12.

**Question 10.**In a potato race a bucket is placed at the starting point ft is 5 m away from the first potato. The rest of the potatoes are placed in a straight line each 3. In away from the other. Each competitor starts from the bucket. Picks up the nearest potato and runs back and drops it in the bucket and continues till all potatoes are placed in the bucket What is the total distance covered if 15 potatoes are placed in the race?

If the distance covered is 1340 m, find the number of potatoes?

**Answer:**From the given data we calculate the distance covered for each potato.

So, for first potato, distance = 2 × 5 = 10m

For second, distance = 10 + 2 × 3= 16m

For third, distance = 16 + 2 × 3= 22m

Thus the distance to be covered form an AP. :

10, 16, 22, 28, …………

The total distance to be covered for 15 potatoes is given by S_{15} .

Hence, for the above AP, we have a = 10 and d = 6.

So, we know that, S_{n} = × (2a + (n – 1)d)

⇒ S_{15}= × [2(10) + (15 – 1) × 6]

= × [20 + 14 × 6]

= × [20 + 84]

= × 104

= 15 × 52

= 780m

∴ if 15 potatoes are placed in the race, the total distance covered is 780m.

Now, it is given that, the total distance covered is 1340m and we need to find the no. Of potatoes.

So, let the no. Of potatoes be n, then we take,

⇒ S_{n} = 1340

⇒ 1340 = × (2a + (n – 1)d)

⇒ 1340 = × (2(10) + (n – 1)(6))

⇒ 1340 = × (20 + 6n – 6)

⇒ 1340 = × (14 + 6n)

⇒ 1340 = n × (7 + 3n)

⇒ 3n^{2} + 7n – 1340 = 0

On solving we get that,

⇒ n = or n = 20

as we n cannot be negative, so we have n = 20.

∴ if total distance to be covered is 1340m, then no. Of potatoes placed in the race are 20.

**Question 11.**A ladder has rungs 25 cm apart. The rungs decrease uniformly from 60 cm at bottom to 40 cm at top. If the distance between the top rung and the bottom rung is 2.5 in, find the length of the wood required.

**Answer:**The distance between 2 consecutive rungs is 25cm and the distance between the top and bottom rung is 2.5m = 250cm.

∴ no. Of rungs = + 1 = 11

The length of the bottom rung is 60cm and going upwards the length of the rung decreases uniformly.

The length of last rung is 40cm.

So, the length of the rung will form a finite AP.

With first term a = 60(T_{1}) and the last 11^{th} term as 40(T_{11})

Now, d =

=

=

⇒ d= – 2

the length of the wood required is given by S_{11}

we know that, S_{n} = × (2a + (n – 1)d)

So, S_{11} = × (2a + (n – 1)d)

⇒ S_{11} = × (2(60) + (11 – 1)( – 2))

⇒ S_{11} = × ( 120 – 20)

⇒ S_{11} = × 100

⇒ S_{11} = 550

∴ the length of the wood required is 550cm i.e. 5.5m.

**Question 12.**A man purchased LCD TV for Rs. 32, 500. He paid Rs. 200 initially and increasing the payment by Rs. 150 every month. How many months did he take to make the complete payment?

**Answer:**The mount paid as down payment is 200Rs.

Amount paid in 1^{st} instalment = 200 + 150 = 350Rs

Amount paid in 2^{nd} instalment = 350 + 150 = 500Rs and so on.

The amount paid every month increase every month and forms finite AP. Which is : 200, 350, 500 ……..

Total amount paid is S_{n} = 32, 500Rs

we know that, S_{n} = × (2a + (n – 1)d)

⇒ 32, 500 = × (2(200) + (n – 1)(150))

⇒ 32, 500 = × (400 + 150n – 150)

⇒ 32, 500 = × (250 + 150n)

⇒ 32, 500 =n × (125 + 75n)

⇒ 75n^{2} + 125n – 32500 = 0

⇒ 25n^{2} + 5n – 1300 = 0

⇒ (3n + 65)(n – 20)=0

⇒ n = or n = 20

As n cannot be negative, so we have n = 20.

Thus we can say that there are 20 terms in the AP.

Among these terms first payment is the down payment, so man took 20 – 1 = 19 months to complete the payment.

∴ man took 19 months to complete the payment.

**Question 13.**In an A.P, T_{1}= 22, T_{n} = —11, S_{n} = 66, find n.

**Answer:**We know that, T_{n} = a + (n – 1)d

So for T_{1,}

We have T_{1}=a + (1 – 1)d = a

According to the question, a = 22

Now, we have T_{n} = – 11

Ie., a + (n – 1)d = – 11

So, 22 + (n – 1)d = – 11

⇒ (n – 1)d = – 22 – 11

⇒ (n – 1)d = – 33

Now we have that, S_{n} = × (2a + (n – 1)d)

From the question we can say that, S_{n} = 66

So, we have,

⇒ 66= × (2a + (n – 1)d)

⇒ 132 = n × (2a + (n – 1)d)

We have (n – 1)d = – 33 and a = 22, so we put that in the above equation.

⇒ 132 = n × (2(22) – 33)

⇒ 132 = n × (44 – 33)

⇒ 132= n × 11

⇒ n =

⇒ n = 12

∴ value of n is 12.

**Question 14.**In an A.P. a = 8, T_{n}= 33, S_{n} = 123, find d and n.

**Answer:**We have a = 8

We know that, T_{n} = a + (n – 1)d

And from the question we can say that, T_{n}= 53

So we get,

⇒ 33 = a + (n – 1)d

Putting value of a in the above equation,

⇒ 33 = 8 + (n – 1)d

⇒ (n – 1)d = 33 – 8

⇒ (n – 1)d = 25

Now we have that, S_{n} = × (2a + (n – 1)d)

From the question we can say that, S_{n} = 123

So, we have,

⇒ 123 = × (2a + (n – 1)d)

⇒ 246 = n × (2a + (n – 1)d)

now we have a = 8 and (n – 1)d = 25, so we put them in the above equation,

⇒ 246 = n × (2(8) + 25)

⇒ 246 = n × (16 + 25)

⇒ 246 = n × (31)

⇒ n= = 6

⇒ n = 6

now, we know that, (n – 1)d = 25

we put the value of n in the above equation, we get

⇒ (6 – 1)d = 25

⇒ 5d = 25

⇒ d = 5

∴ for the given A.P, value of n is 6 and value of d is 5.

**Question 15.**If T_{3} = 8, T_{7} = 24, then T_{10} = _____

A. —4

B. 28

C. 32

D. 36

**Answer:**We have, T_{3} = 8

And recall that, T_{n} = a + (n – 1)d

So we have, T_{3} = a + (3 – 1)d

⇒ 8 = a + 2d ……(1)

We also have, T_{7} = 24

And T_{7} = a + (7 – 1)d

⇒ 24 = a + 6d …… (2)

From (1), we have that, a = 8 – 2d

So we put the value of a in (2)

⇒ 24 = 8 – 2d + 6d

⇒ 24 – 8 = 4d

⇒ 16 = 4d

⇒ d= 4

now, putting value of d in (1),

⇒ 8 = a + 2(4)

⇒ 8 = a + 8

⇒ a =0

now, T_{10}= a + (10 – 1)d

⇒ T_{10}= 0 + 9 × 4

⇒ T_{10}= 36

∴ correct option is (d).

**Question 16.**If S_{n} = 2n^{2} + 3n, then d =_____

A. 13

B. 4

C. 9

D. —2

**Answer:**We have S_{n} = 2n^{2} + 3n from the question.

We know that, S_{n} = × (2a + (n – 1)d)

So we can say that,

⇒ 2n^{2} + 3n = × (2a + (n – 1)d)

⇒ 4n^{2} + 6n = n × (2a + (n – 1)d)

⇒ 4n^{2} + 6n = 2na + dn^{2} – dn

⇒ 4n^{2} + 6n = dn^{2} + (2a – d)n

Now comparing the coefficients of “n^{2”} and “n”, we get that,

d = 4

∴ the correct option is (b).

**Question 17.**If the sum of the three consecutive terms of A.P. is 48 and the product of the first and the last is 252, then d = _____

A. 2

B. 3

C. 4

D. 16

**Answer:**We know that the sum of 3 nos in the AP is 48.

Lets say that those 3 nos are : a – d, a, a + d

So we can say that, a – d + a + a + d = 48

3a = 48

a= 16

now we have product of the first and the last term out of these 3 to be 252.

Ie. (a – d)(a + d) = 252

⇒ a^{2} – d^{2} = 252

we have a = 16, so we put the value of a in the above equation:

⇒ (16)^{2} – d^{2} = 252

⇒ 256 – d^{2} = 252

⇒ d^{2} = 256 – 252

⇒ d^{2} = 4

⇒ d=2

∴ the correct option is (a)

**Question 18.**If a = 2 and d = 4, then S_{20} = _____

A. 600

B. 800

C. 78

D. 80

**Answer:**We have a = 2

d= 4

n = 20

we also know that, S_{n} = × (2a + (n – 1)d)

⇒ S_{20} = × (2(2) + (20 – 1)4)

⇒ S_{20} = 10 × (4 + 19 × 4)

⇒ S_{20} = 10 × (4 + 76)

⇒ S_{20} = 10 × 80

⇒ S_{20} = 800

∴ the correct option is (b)

**Question 19.**If 3 + 5 + 7 + 9 + ... upto n terms = 288, then n = ……

A. 12

B. 15

C. 16

D. 17

**Answer:**We have a = 3

Then d = 5 – 3 = 2

Then, S_{n} = 288

We can recall that, S_{n} = × (2a + (n – 1)d)

So, S_{n} = × (2(3) + (n – 1)2)

⇒ S_{n} = × (6 + 2n – 2)

⇒ S_{n} = × (4 + 2n)

⇒ S_{n} = n × (2 + n)

⇒ S_{n} = n^{2} + 2n

⇒ n^{2} + 2n = 288

⇒ n^{2} + 2n – 288 = 0

⇒ n^{2} + 18n – 16n – 288 = 0

⇒ n(n + 18) – 16(n + 18) = 0

⇒ (n + 18)(n – 16)= 0

So we have n = – 18 or 16

But as n cannot be negative so, we have n = 16

∴ the correct option is (c)

**Question 20.**Four numbers are in A.P. and their sum is 72 and the largest of them is twice the smallest. Then the numbers are ………

A. 4, 8, 12, 16

B. 12, 16, 20, 24

C. 10, 12, 14, 16

D. 2, 4, 6, 8

**Answer:**We have n = 4

⇒ S_{4} = 72

Let the 4 nos. Be, a, a + d, a + 2d, a + 3d

Now we have that a + a + d + a + 2d + a + 3d = 72

⇒ 4a + 6d = 72

⇒ 2a + 3d = 36 …….(1)

Now We know that the largest of them is twice the smallest.

So we can say that,

⇒ 2a = a + 3d

⇒ a = 3d

we now put the value of a in (1),

to get :

⇒ 2a + a = 36

⇒ 3a = 36

⇒ a = 12

we know that a is the first term in this AP, so the possible option is (b)

but to check whether it is the correct we will have to check the relation between the first and the last term of the AP of (b) option.

We can see that, in (b), first term is 12

And the last term is 24 so, this is the correct AP.

∴ the correct option is (b)

**Question 21.**If S_{1}= 2 + 4 + ... + 2n and S_{2} = 1 + 3 + ... + (2n — 1), then S_{1}: S_{2} = …..

A.

B.

C. n^{2}

D. (n + 1)

**Answer:**We know that, S_{n} = × (a + l)

Where a = first term of the AP.

And l = last term of the AP

For S_{1,}

We have S_{1} = × (2 + 2n)

⇒ S_{1} = n × (n + 1)

For S_{2,}

We have S_{2} = × (1 + 2n – 1)

⇒ S_{2} = × (2n)

⇒ S_{2} = n^{2}

Now = =

∴ =

∴ the correct option is (a)

**Question 22.**For A.P., S_{n} — 2S_{n – 1} + S_{n – 2} =…. (n > 2)

A. 2d

B. d

C. a

D. a + d

**Answer:**We can recall that, S_{n} = × (2a + (n – 1)d) ……(1)

So now, S_{n – 1} = × (2a + ((n – 1) – 1)d)

⇒ S_{n – 1} = × (2a + (n – 2)d) …….(2)

So now, S_{n – 2} = × (2a + ((n – 2) – 1)d)

⇒ S_{n – 2} = × (2a + (n – 3)d) ………(3)

Now putting the above values in the equation_{,}

We get,

⇒ S_{n} — 2S_{n – 1} + S_{n – 2}

= × (2a + (n – 1)d) – 2[ × (2a + (n – 2)d)] + × (2a + (n – 3)d)

= – 2[] +

= an + – – 2an – dn^{2} + 2a – 2d + 3nd + an + – – 2a + 3d

= [an – 2an + an] + [ – dn^{2} + ] + [2a – 2a] + [ – + 3nd – ] + [ – 2d + 3d]

= 0 + 0 + 0 + 0 + d

= d

∴ S_{n} — 2S_{n – 1} + S_{n – 2} = d

∴ the correct option is (b)

**Question 23.**If S_{m} = n and S_{n} = m then S_{m + n} = ……

A. —(m + n)

B. 0

C. m + n

D. 2m — 2n

**Answer:**⇒ S_{m} = n = × [ 2a + (m – 1)d ]

⇒ = 2a + (m – 1)d ......................(1)

⇒ S_{n} = m = × [ 2a + (n – 1)d ]

⇒ = 2a + (n – 1)d ........................(2)

subtracting both equations, we get :

⇒ 2( – ) = d(m – n)

⇒ d = – 2[] ...................(3)

now, S_{m + n} = × [ 2a + (m + n – 1)d ]

⇒ S_{m + n} × = 2a + (m + n – 1)d .......................(4)

now, (4) – (2), we get :

⇒ S_{m + n} × – = d(m)

putting value of d from (3), we get :

⇒ S_{m + n} × – = – 2[] × m

⇒ S_{m + n} × = – 2[] × m +

⇒ S_{m + n} × = – 2[] +

⇒ S_{m + n} × = – 2 or

⇒ S_{m + n} = – (m + n)

∴ the correct option is (a).

**Question 24.**If T_{4} = 7 and T_{7} = 4, then T_{10} = …..

A. 9

B. 11

C. —11

D. 1

**Answer:**Given T_{4} = 7

And we know that, T_{n} = a + (n – 1)d

So we have T_{4} = a + (4 – 1)d

⇒ 7 = a + 3d …… (1)

Then, T_{7} = 4

⇒ T_{7} = a + (7 – 1)d

⇒ 4 = a + 6d …….(2)

Now, (2) – (1) gives :

⇒ – 3 = 3d

⇒ d = – 1

so, now, putting value of d in (1),

⇒ 7 = a + 3d

⇒ 7 = a + 3( – 1)

⇒ 7 = a – 3

⇒ a = 10

so now T_{10} = a + (10 – 1)d

⇒ T_{10} = a + 9d

= 10 + 9( – 1)

= 10 – 9

= 1

∴ T_{10} = 1

∴ correct option is (d)

**Question 25.**If 2k + 1, 13, 5k — 3 are three consecutive terms of A.P., then k = ……….

A. 17

B. 13

C. 4

D. 9

**Answer:**If 2k + 1, 13, 5k — 3 are three consecutive terms of A.P., then,

⇒ 13 – (2k + 1) = (5k – 3 ) – 13 …..(common difference)

⇒ 12 – 2k = 5k – 16

⇒ 28 = 7k

⇒ k = 4

∴ correct option is (c).

**Question 26.**(1) + (1 + 1) + (1 + 1 + 1) + ... + (1 + 1 + 1 + ...n — 1 times) =

A.

B.

C. n

D. n^{2}

**Answer:**(1) + (1 + 1) + (1 + 1 + 1) + .…. + (1 + 1 + 1 + ...n — 1 times)

= 1 + 2 + 3 + 4 ………… + (n – 1)

= S_{n – 1}

⇒ S_{n – 1} = × [2a + (n – 1 – 1)d]

= × [2(1) + (n – 2)(1)]

=

=

=

=

∴ the correct option is (a).

**Question 27.**In the A.P., 5, 7, 9, 11, 13, 15, ... the sixth term which is prime is

A. 13

B. 19

C. 23

D. 15

**Answer:**If we see that, in the given series, d = 2

So if e continue the series, we get :

5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25 …….

We can see that, the prime nos. In the above are :

5, 7, 11, 13, 17, 19, 23, …

so here we see that the sixth term which is prime is 19

∴ the correct option is (b)

**Question 28.**For A.P. T_{18} — T_{8} = ………

A. d

B. 10d

C. 26d

D. 2d

**Answer:**We know that, T_{n} = a + (n – 1)d

So we have, T_{18}= a + (18 – 1)d

⇒ T_{18}= a + 17d

Now T_{8}= a + (8 – 1)d

⇒ T_{8}= a + 7d

Now we have,

⇒ T_{18} — T_{8} = [a + 17d] – [a + 7d]

= 10d

∴ the correct option is (b)

**Question 29.**If for A.P., T_{25} — T_{20} = 15 then d =

A. 3

B. 5

C. 20

D. 25

**Answer:**We know that, T_{n} = a + (n – 1)d

So we have, T_{25}= a + (25 – 1)d

⇒ T_{25}= a + 24d

Now we have, T_{20}= a + (20 – 1)d

⇒ T_{20}= a + 19d

It is given that, T_{25} — T_{20} = 15

So we can say that,

⇒ 15 = [a + 24d] – [a + 19d]

⇒ 15 = 5d

⇒ d = 3

∴ the correct option is (a).

**Question 1.**

If T_{n} = 6n + 5, find S_{n}

**Answer:**

From the question we know that, T_{n} = 6n + 5 ……. (1)

And we know that T_{n} = a + (n – 1)d

So we have,

⇒ a + (n – 1)d = 6n + 5

⇒ a + (n – 1)d = 6n + 11 – 6

⇒ a + (n – 1)d = 11 + (6n – 6)

⇒ a + (n – 1)d = 11 + 6(n – 1)

On comparing both the sides, we have:

⇒ a = 11 and d = 6

So now, S_{n} = × (2a + (n – 1)d)

⇒ S_{n} = × (2(11) + (n – 1)(6))

⇒ S_{n} = × (22 + 6n – 6)

⇒ S_{n} = × (16 + 6n)

⇒ S_{n} = n × (8 + 3n)

⇒ S_{n} = 3n^{2} + 8n

∴ Sum of n terms of the given A.P is S_{n} = 3n^{2} + 8n

**Question 2.**

If S_{n} = n^{2} + 2n, find T_{n}

**Answer:**

From the question we know that, S_{n} = n^{2} + 2n ……. (1)

We know that, S_{n} = × (2a + (n – 1)d)

= na +

= na + (n^{2} – n) ×

S_{n} = na + n^{2} × – n × ……. (2)

As we know that, (1) and (2) are both sum of the same arithmetic progression

So we can equate them to each other.

So, we have,

⇒ n^{2} + 2n = na + n^{2} × – n ×

⇒ n^{2} + 2n = n^{2} × + na – n ×

Now we will equate the coefficients of “n^{2}” and “n” on both the sides of the equal to sign.

So, we get,

For coefficients of n^{2} :

⇒ 1 =

⇒ d=2

Now, For coefficients of n:

⇒ 2 = a –

From above we have that d=2,

So we can say that,

⇒ 2 = a –

⇒ 2 = a – 1

⇒ a=3

So, now, we know that, T_{n} = a + (n – 1)d

Now we put values of a and n in the above equation,

We get,

⇒ T_{n} = 3 + (n – 1)2

⇒ T_{n} = 3 + 2n – 2

⇒ T_{n} = 2n + 1

∴ for the given value of S_{n} = n^{2} + 2n, we have T_{n} = 2n + 1.

**Question 3.**

If the sum of the first n terms of A, P.

30, 27, 24, 21 … is 120, find the number of terms and the last term.

**Answer:**

We can see that, A.P. is 30, 27, 24, 21 …

T_{1} = 30 = a

T_{2} = 27

So, we have d = T_{2} – T_{1}

d = 27 – 30

d= – 3

Now, we have sum of the A.P. S_{n} = × (2a + (n – 1)d)

And it is given that, S_{n} = 120

So, we have,

⇒ 120 = × (2a + (n – 1)d)

Now we will put the values of a and d in the above equation

⇒ 120 = × (2(30) + (n – 1)(– 3))

⇒ 120 = × (60 + 3 – 3n)

120 × 2 = n × (63 – 3n)

⇒ 240 = 63n – 3n^{2}

⇒ 3n^{2} – 63n + 240 = 0

⇒ n^{2} – 21n + 80 = 0

⇒ n^{2} – 16n – 5n + 80 = 0

⇒ n(n – 16) – 5(n – 16) = 0

⇒ (n – 16)(n – 5) = 0

⇒ n= 16 or 5

Now, when n=16, the last term = T_{16}

So we have, T_{16} = a + (16 – 1)d ……. (∵ T_{n} = a + (n – 1)d )

⇒ T_{16} = a + 15d

Now we put the values of a and d in the above equation

⇒ T_{16}= 30 + 15(– 3)

⇒ T_{16} = 30 – 45

⇒ T_{16} = – 15

Now, when n=5, the last term = T_{5}

So we have, T5 = a + (5 – 1)d ……. (∵ Tn = a + (n – 1)d )

⇒ T_{5} = a + 4d

Now we put the values of a and d in the above equation

⇒ T_{5}= 30 + 4(– 3)

⇒ T_{5} = 30 – 12

⇒ T_{5} = 18

∴ The no. Of term in the above A.P. can be n = 5 or 16

And the last terms of the respective A.Ps are:

For n = 5, last term T5 = 18

For n = 16, last term T16 = – 15

**Question 4.**

Which term of A.P., 100, 97, 94, 91, will be its first —ve term?

**Answer:**

We have the A.P as 100, 97, 94, 91 …

So, a = 100

⇒ d= 97 – 100 = – 3

Now recall that, nth term of an AP is T_{n} = a + (n – 1)d

⇒ T_{n} = 100 + (n – 1)(– 3)

= 100 – 3n + 3

⇒ T_{n} = 103 – 3n

Since we need negative term, T_{n} < 0

So, 103 – 3n < 0

⇒ 103 < 3n

⇒ < n

⇒ 34.333 < n

⇒ n>34

i.e. N = 35

∴ n=35 for the first negative term in the given A.P.

⇒ T_{35} = a + (35 – 1)d

= 100 + 34 × – 3

= 100 – 102

= – 2

∴ the first negative term of the given AP is – 2.

**Question 5.**

Find the sum of all 3 digit natural multiples of 6.

**Answer:**

Now the first 3 digit multiple of 6 is 102.

That means, a = 102

And all the nos. Are multiples of 6 which means that they have a difference of 6 in between them so, common difference d = 6

Now, the last 3 digit multiple of 6 is 996.

Now, we know that, T_{n} = a + (n – 1)d

So, 996 = a + (n – 1)d

⇒ 996 = 102 + (n – 1)6

⇒ 6(n – 1) = 996 – 102

⇒ 6(n – 1) = 894

⇒ n – 1 =

⇒ n – 1 = 149

⇒ n = 150

So, we can say that there are 150, 3 digit multiples of 6

Now sum of these nos. S_{150} = × (2a + (n – 1)d)

⇒ S_{150} = × (2(102) + (150 – 1)6)

= 75 × (204 + 149 × 6)

= 75 × (1098)

⇒ S_{150} = 82350

∴ Sum of all 3 digit natural multiples of 6 is 82350.

**Question 6.**

The ratio of the sum of m terms to sum of n terms of an A.P. is. Find the ratio of its mth term to its nth term.

**Answer:**

As we know that sum of n terms of an A.P is:

⇒ S_{n}= × (2a + (n – 1)d)

So, sum of m terms of an A.P is:

⇒ S_{m} = × (2a + (m – 1)d)

From the question we know that,

⇒ =

⇒ =

⇒ =

⇒ =

⇒ m × (2a + (n – 1)d) = n × (2a + (m – 1)d)

⇒ 2ma + m(n – 1)d = 2na + n(m – 1)d

⇒ 2ma + md(n – 1) = 2na + nd(m – 1)

⇒ 2ma + mnd – md = 2na + mnd – nd

⇒ 2ma – md = 2na – nd

⇒ 2ma – 2na = md – nd

⇒ 2a(m – n) = d(m – n)

⇒ 2a = d

Now, m^{th} term of the given A.P is T_{m} = a + (m – 1)d

Now, n^{th} term of the given A.P is T_{n} = a + (n – 1)d

Now ratio between these 2 terms is:

⇒ =

Now we have, 2a = d

So we get,

⇒ =

⇒ =

⇒ =

⇒ =

⇒ =

∴ the ratio of m^{th} term of the given A.P to its n^{th} Term is:

T_{m} : T_{n} = 2m – 1 : 2n – 1

**Question 7.**

Sum to first l, m, n terms of A.P. are p, q, r. Prove that

**Answer:**

Let the first term of AP be a and common difference be d.

Sum of the first p terms is:

S_{l} = × (2a + (l – 1)d) = p ………(1)

Sum of the first m terms is:

⇒ S_{m} = × (2a + (m – 1)d) = q ……… (2)

Sum of the first n terms is:

⇒ S_{n} = × (2a + (n – 1)d) = r ……… (3)

Now, (1) × + (2) × + (3) ×

We get, p × + q × + r ×

= 0 + [(m – n)l + (n – l)m + (l – m)n –m + n – n + l – l + m]

= [lm – ln + mn – lm + ln – mn + 0]

= × 0 = 0

∴ it is proved that, p × + q × + r × = 0

**Question 8.**

The ratio of sum to n terms of two A.P. is for every n ∈ N. Find the ratio of their 7^{th} terms and m^{th} terms.

**Answer:**

Let the sum of n terms of the first A.P be:

⇒ S_{n} = × (2a + (n – 1)d) …………… (1)

Let the sum of n terms of the second A.P be:

⇒ S’_{n} = × (2a’ + (n – 1)d’) …………… (2)

Now according to the question:

⇒ =

Let’s consider the ratio these two AP’s m^{th} terms as:

T_{m} : T’_{m}

Now, recall that, nth term of an AP is T_{n} = a + (n – 1)d

⇒ T_{m} = a + (m – 1)d

⇒ T’^{m} = a’ + (m – 1)d’

Hence the ratio of these two AP’s m^{th} terms become:

⇒ =

On multiplying by 2, we get,

⇒ =

⇒ =

⇒ =

⇒ =

⇒ =

⇒ =

Now from the above formula of the ratio of mth terms of 2 Aps, we can find the ratio of 7^{th} terms of both Aps

So we have, =

⇒ =

⇒ =

∴ the ratio of m^{th} terms of the given 2 Aps is, 16m – 7 : 14m – 4

∴ the ratio of 7^{th} terms of the given 2 Aps is, 105 : 94

**Question 9.**

Three numbers in A.P. have the sum 18 and the sum of their squares is 180. Find the numbers in the increasing order.

**Answer:**

We know that the sum of 3 numbers in the AP is 18.

Let’s say that those 3 numbers are: a – d, a, a + d

So we can say that, a – d + a + a + d = 18

⇒ 3a = 18

⇒ a= 6

Now, we have that the sum of squares of these 3 numbers is 180.

So we can say that, (a – d) ^{2} + a^{2} + (a + d) ^{2} = 180

⇒ a^{2} – 2ad + d^{2} + a^{2} + a^{2} + 2ad + d^{2} = 180

⇒ 3a^{2} + 2d^{2} = 180

We know that a = 6,

So, 3(6)^{2} + 2d^{2} = 180

⇒ 3(36) + 2d^{2} = 180

⇒ 108 + 2d^{2} = 180

⇒ 2d^{2} = 180 – 108

⇒ 2d^{2} = 72

⇒ d^{2} = 36

⇒ d = 6

Now, our 3 numbers a – d, a, a + d are 6 – 6, 6, 6 + 6 = 0, 6, 12 respectively.

∴ The 3 numbers of the A.P in the increasing order are : 0, 6, 12.

**Question 10.**

In a potato race a bucket is placed at the starting point ft is 5 m away from the first potato. The rest of the potatoes are placed in a straight line each 3. In away from the other. Each competitor starts from the bucket. Picks up the nearest potato and runs back and drops it in the bucket and continues till all potatoes are placed in the bucket What is the total distance covered if 15 potatoes are placed in the race?

If the distance covered is 1340 m, find the number of potatoes?

**Answer:**

From the given data we calculate the distance covered for each potato.

So, for first potato, distance = 2 × 5 = 10m

For second, distance = 10 + 2 × 3= 16m

For third, distance = 16 + 2 × 3= 22m

Thus the distance to be covered form an AP. :

10, 16, 22, 28, …………

The total distance to be covered for 15 potatoes is given by S_{15} .

Hence, for the above AP, we have a = 10 and d = 6.

So, we know that, S_{n} = × (2a + (n – 1)d)

⇒ S_{15}= × [2(10) + (15 – 1) × 6]

= × [20 + 14 × 6]

= × [20 + 84]

= × 104

= 15 × 52

= 780m

∴ if 15 potatoes are placed in the race, the total distance covered is 780m.

Now, it is given that, the total distance covered is 1340m and we need to find the no. Of potatoes.

So, let the no. Of potatoes be n, then we take,

⇒ S_{n} = 1340

⇒ 1340 = × (2a + (n – 1)d)

⇒ 1340 = × (2(10) + (n – 1)(6))

⇒ 1340 = × (20 + 6n – 6)

⇒ 1340 = × (14 + 6n)

⇒ 1340 = n × (7 + 3n)

⇒ 3n^{2} + 7n – 1340 = 0

On solving we get that,

⇒ n = or n = 20

as we n cannot be negative, so we have n = 20.

∴ if total distance to be covered is 1340m, then no. Of potatoes placed in the race are 20.

**Question 11.**

A ladder has rungs 25 cm apart. The rungs decrease uniformly from 60 cm at bottom to 40 cm at top. If the distance between the top rung and the bottom rung is 2.5 in, find the length of the wood required.

**Answer:**

The distance between 2 consecutive rungs is 25cm and the distance between the top and bottom rung is 2.5m = 250cm.

∴ no. Of rungs = + 1 = 11

The length of the bottom rung is 60cm and going upwards the length of the rung decreases uniformly.

The length of last rung is 40cm.

So, the length of the rung will form a finite AP.

With first term a = 60(T_{1}) and the last 11^{th} term as 40(T_{11})

Now, d =

=

=

⇒ d= – 2

the length of the wood required is given by S_{11}

we know that, S_{n} = × (2a + (n – 1)d)

So, S_{11} = × (2a + (n – 1)d)

⇒ S_{11} = × (2(60) + (11 – 1)( – 2))

⇒ S_{11} = × ( 120 – 20)

⇒ S_{11} = × 100

⇒ S_{11} = 550

∴ the length of the wood required is 550cm i.e. 5.5m.

**Question 12.**

A man purchased LCD TV for Rs. 32, 500. He paid Rs. 200 initially and increasing the payment by Rs. 150 every month. How many months did he take to make the complete payment?

**Answer:**

The mount paid as down payment is 200Rs.

Amount paid in 1^{st} instalment = 200 + 150 = 350Rs

Amount paid in 2^{nd} instalment = 350 + 150 = 500Rs and so on.

The amount paid every month increase every month and forms finite AP. Which is : 200, 350, 500 ……..

Total amount paid is S_{n} = 32, 500Rs

we know that, S_{n} = × (2a + (n – 1)d)

⇒ 32, 500 = × (2(200) + (n – 1)(150))

⇒ 32, 500 = × (400 + 150n – 150)

⇒ 32, 500 = × (250 + 150n)

⇒ 32, 500 =n × (125 + 75n)

⇒ 75n^{2} + 125n – 32500 = 0

⇒ 25n^{2} + 5n – 1300 = 0

⇒ (3n + 65)(n – 20)=0

⇒ n = or n = 20

As n cannot be negative, so we have n = 20.

Thus we can say that there are 20 terms in the AP.

Among these terms first payment is the down payment, so man took 20 – 1 = 19 months to complete the payment.

∴ man took 19 months to complete the payment.

**Question 13.**

In an A.P, T_{1}= 22, T_{n} = —11, S_{n} = 66, find n.

**Answer:**

We know that, T_{n} = a + (n – 1)d

So for T_{1,}

We have T_{1}=a + (1 – 1)d = a

According to the question, a = 22

Now, we have T_{n} = – 11

Ie., a + (n – 1)d = – 11

So, 22 + (n – 1)d = – 11

⇒ (n – 1)d = – 22 – 11

⇒ (n – 1)d = – 33

Now we have that, S_{n} = × (2a + (n – 1)d)

From the question we can say that, S_{n} = 66

So, we have,

⇒ 66= × (2a + (n – 1)d)

⇒ 132 = n × (2a + (n – 1)d)

We have (n – 1)d = – 33 and a = 22, so we put that in the above equation.

⇒ 132 = n × (2(22) – 33)

⇒ 132 = n × (44 – 33)

⇒ 132= n × 11

⇒ n =

⇒ n = 12

∴ value of n is 12.

**Question 14.**

In an A.P. a = 8, T_{n}= 33, S_{n} = 123, find d and n.

**Answer:**

We have a = 8

We know that, T_{n} = a + (n – 1)d

And from the question we can say that, T_{n}= 53

So we get,

⇒ 33 = a + (n – 1)d

Putting value of a in the above equation,

⇒ 33 = 8 + (n – 1)d

⇒ (n – 1)d = 33 – 8

⇒ (n – 1)d = 25

Now we have that, S_{n} = × (2a + (n – 1)d)

From the question we can say that, S_{n} = 123

So, we have,

⇒ 123 = × (2a + (n – 1)d)

⇒ 246 = n × (2a + (n – 1)d)

now we have a = 8 and (n – 1)d = 25, so we put them in the above equation,

⇒ 246 = n × (2(8) + 25)

⇒ 246 = n × (16 + 25)

⇒ 246 = n × (31)

⇒ n= = 6

⇒ n = 6

now, we know that, (n – 1)d = 25

we put the value of n in the above equation, we get

⇒ (6 – 1)d = 25

⇒ 5d = 25

⇒ d = 5

∴ for the given A.P, value of n is 6 and value of d is 5.

**Question 15.**

If T_{3} = 8, T_{7} = 24, then T_{10} = _____

A. —4

B. 28

C. 32

D. 36

**Answer:**

We have, T_{3} = 8

And recall that, T_{n} = a + (n – 1)d

So we have, T_{3} = a + (3 – 1)d

⇒ 8 = a + 2d ……(1)

We also have, T_{7} = 24

And T_{7} = a + (7 – 1)d

⇒ 24 = a + 6d …… (2)

From (1), we have that, a = 8 – 2d

So we put the value of a in (2)

⇒ 24 = 8 – 2d + 6d

⇒ 24 – 8 = 4d

⇒ 16 = 4d

⇒ d= 4

now, putting value of d in (1),

⇒ 8 = a + 2(4)

⇒ 8 = a + 8

⇒ a =0

now, T_{10}= a + (10 – 1)d

⇒ T_{10}= 0 + 9 × 4

⇒ T_{10}= 36

∴ correct option is (d).

**Question 16.**

If S_{n} = 2n^{2} + 3n, then d =_____

A. 13

B. 4

C. 9

D. —2

**Answer:**

We have S_{n} = 2n^{2} + 3n from the question.

We know that, S_{n} = × (2a + (n – 1)d)

So we can say that,

⇒ 2n^{2} + 3n = × (2a + (n – 1)d)

⇒ 4n^{2} + 6n = n × (2a + (n – 1)d)

⇒ 4n^{2} + 6n = 2na + dn^{2} – dn

⇒ 4n^{2} + 6n = dn^{2} + (2a – d)n

Now comparing the coefficients of “n^{2”} and “n”, we get that,

d = 4

∴ the correct option is (b).

**Question 17.**

If the sum of the three consecutive terms of A.P. is 48 and the product of the first and the last is 252, then d = _____

A. 2

B. 3

C. 4

D. 16

**Answer:**

We know that the sum of 3 nos in the AP is 48.

Lets say that those 3 nos are : a – d, a, a + d

So we can say that, a – d + a + a + d = 48

3a = 48

a= 16

now we have product of the first and the last term out of these 3 to be 252.

Ie. (a – d)(a + d) = 252

⇒ a^{2} – d^{2} = 252

we have a = 16, so we put the value of a in the above equation:

⇒ (16)^{2} – d^{2} = 252

⇒ 256 – d^{2} = 252

⇒ d^{2} = 256 – 252

⇒ d^{2} = 4

⇒ d=2

∴ the correct option is (a)

**Question 18.**

If a = 2 and d = 4, then S_{20} = _____

A. 600

B. 800

C. 78

D. 80

**Answer:**

We have a = 2

d= 4

n = 20

we also know that, S_{n} = × (2a + (n – 1)d)

⇒ S_{20} = × (2(2) + (20 – 1)4)

⇒ S_{20} = 10 × (4 + 19 × 4)

⇒ S_{20} = 10 × (4 + 76)

⇒ S_{20} = 10 × 80

⇒ S_{20} = 800

∴ the correct option is (b)

**Question 19.**

If 3 + 5 + 7 + 9 + ... upto n terms = 288, then n = ……

A. 12

B. 15

C. 16

D. 17

**Answer:**

We have a = 3

Then d = 5 – 3 = 2

Then, S_{n} = 288

We can recall that, S_{n} = × (2a + (n – 1)d)

So, S_{n} = × (2(3) + (n – 1)2)

⇒ S_{n} = × (6 + 2n – 2)

⇒ S_{n} = × (4 + 2n)

⇒ S_{n} = n × (2 + n)

⇒ S_{n} = n^{2} + 2n

⇒ n^{2} + 2n = 288

⇒ n^{2} + 2n – 288 = 0

⇒ n^{2} + 18n – 16n – 288 = 0

⇒ n(n + 18) – 16(n + 18) = 0

⇒ (n + 18)(n – 16)= 0

So we have n = – 18 or 16

But as n cannot be negative so, we have n = 16

∴ the correct option is (c)

**Question 20.**

Four numbers are in A.P. and their sum is 72 and the largest of them is twice the smallest. Then the numbers are ………

A. 4, 8, 12, 16

B. 12, 16, 20, 24

C. 10, 12, 14, 16

D. 2, 4, 6, 8

**Answer:**

We have n = 4

⇒ S_{4} = 72

Let the 4 nos. Be, a, a + d, a + 2d, a + 3d

Now we have that a + a + d + a + 2d + a + 3d = 72

⇒ 4a + 6d = 72

⇒ 2a + 3d = 36 …….(1)

Now We know that the largest of them is twice the smallest.

So we can say that,

⇒ 2a = a + 3d

⇒ a = 3d

we now put the value of a in (1),

to get :

⇒ 2a + a = 36

⇒ 3a = 36

⇒ a = 12

we know that a is the first term in this AP, so the possible option is (b)

but to check whether it is the correct we will have to check the relation between the first and the last term of the AP of (b) option.

We can see that, in (b), first term is 12

And the last term is 24 so, this is the correct AP.

∴ the correct option is (b)

**Question 21.**

If S_{1}= 2 + 4 + ... + 2n and S_{2} = 1 + 3 + ... + (2n — 1), then S_{1}: S_{2} = …..

A.

B.

C. n^{2}

D. (n + 1)

**Answer:**

We know that, S_{n} = × (a + l)

Where a = first term of the AP.

And l = last term of the AP

For S_{1,}

We have S_{1} = × (2 + 2n)

⇒ S_{1} = n × (n + 1)

For S_{2,}

We have S_{2} = × (1 + 2n – 1)

⇒ S_{2} = × (2n)

⇒ S_{2} = n^{2}

Now = =

∴ =

∴ the correct option is (a)

**Question 22.**

For A.P., S_{n} — 2S_{n – 1} + S_{n – 2} =…. (n > 2)

A. 2d

B. d

C. a

D. a + d

**Answer:**

We can recall that, S_{n} = × (2a + (n – 1)d) ……(1)

So now, S_{n – 1} = × (2a + ((n – 1) – 1)d)

⇒ S_{n – 1} = × (2a + (n – 2)d) …….(2)

So now, S_{n – 2} = × (2a + ((n – 2) – 1)d)

⇒ S_{n – 2} = × (2a + (n – 3)d) ………(3)

Now putting the above values in the equation_{,}

We get,

⇒ S_{n} — 2S_{n – 1} + S_{n – 2}

= × (2a + (n – 1)d) – 2[ × (2a + (n – 2)d)] + × (2a + (n – 3)d)

= – 2[] +

= an + – – 2an – dn^{2} + 2a – 2d + 3nd + an + – – 2a + 3d

= [an – 2an + an] + [ – dn^{2} + ] + [2a – 2a] + [ – + 3nd – ] + [ – 2d + 3d]

= 0 + 0 + 0 + 0 + d

= d

∴ S_{n} — 2S_{n – 1} + S_{n – 2} = d

∴ the correct option is (b)

**Question 23.**

If S_{m} = n and S_{n} = m then S_{m + n} = ……

A. —(m + n)

B. 0

C. m + n

D. 2m — 2n

**Answer:**

⇒ S_{m} = n = × [ 2a + (m – 1)d ]

⇒ = 2a + (m – 1)d ......................(1)

⇒ S_{n} = m = × [ 2a + (n – 1)d ]

⇒ = 2a + (n – 1)d ........................(2)

subtracting both equations, we get :

⇒ 2( – ) = d(m – n)

⇒ d = – 2[] ...................(3)

now, S_{m + n} = × [ 2a + (m + n – 1)d ]

⇒ S_{m + n} × = 2a + (m + n – 1)d .......................(4)

now, (4) – (2), we get :

⇒ S_{m + n} × – = d(m)

putting value of d from (3), we get :

⇒ S_{m + n} × – = – 2[] × m

⇒ S_{m + n} × = – 2[] × m +

⇒ S_{m + n} × = – 2[] +

⇒ S_{m + n} × = – 2 or

⇒ S_{m + n} = – (m + n)

∴ the correct option is (a).

**Question 24.**

If T_{4} = 7 and T_{7} = 4, then T_{10} = …..

A. 9

B. 11

C. —11

D. 1

**Answer:**

Given T_{4} = 7

And we know that, T_{n} = a + (n – 1)d

So we have T_{4} = a + (4 – 1)d

⇒ 7 = a + 3d …… (1)

Then, T_{7} = 4

⇒ T_{7} = a + (7 – 1)d

⇒ 4 = a + 6d …….(2)

Now, (2) – (1) gives :

⇒ – 3 = 3d

⇒ d = – 1

so, now, putting value of d in (1),

⇒ 7 = a + 3d

⇒ 7 = a + 3( – 1)

⇒ 7 = a – 3

⇒ a = 10

so now T_{10} = a + (10 – 1)d

⇒ T_{10} = a + 9d

= 10 + 9( – 1)

= 10 – 9

= 1

∴ T_{10} = 1

∴ correct option is (d)

**Question 25.**

If 2k + 1, 13, 5k — 3 are three consecutive terms of A.P., then k = ……….

A. 17

B. 13

C. 4

D. 9

**Answer:**

If 2k + 1, 13, 5k — 3 are three consecutive terms of A.P., then,

⇒ 13 – (2k + 1) = (5k – 3 ) – 13 …..(common difference)

⇒ 12 – 2k = 5k – 16

⇒ 28 = 7k

⇒ k = 4

∴ correct option is (c).

**Question 26.**

(1) + (1 + 1) + (1 + 1 + 1) + ... + (1 + 1 + 1 + ...n — 1 times) =

A.

B.

C. n

D. n^{2}

**Answer:**

(1) + (1 + 1) + (1 + 1 + 1) + .…. + (1 + 1 + 1 + ...n — 1 times)

= 1 + 2 + 3 + 4 ………… + (n – 1)

= S_{n – 1}

⇒ S_{n – 1} = × [2a + (n – 1 – 1)d]

= × [2(1) + (n – 2)(1)]

=

=

=

=

∴ the correct option is (a).

**Question 27.**

In the A.P., 5, 7, 9, 11, 13, 15, ... the sixth term which is prime is

A. 13

B. 19

C. 23

D. 15

**Answer:**

If we see that, in the given series, d = 2

So if e continue the series, we get :

5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25 …….

We can see that, the prime nos. In the above are :

5, 7, 11, 13, 17, 19, 23, …

so here we see that the sixth term which is prime is 19

∴ the correct option is (b)

**Question 28.**

For A.P. T_{18} — T_{8} = ………

A. d

B. 10d

C. 26d

D. 2d

**Answer:**

We know that, T_{n} = a + (n – 1)d

So we have, T_{18}= a + (18 – 1)d

⇒ T_{18}= a + 17d

Now T_{8}= a + (8 – 1)d

⇒ T_{8}= a + 7d

Now we have,

⇒ T_{18} — T_{8} = [a + 17d] – [a + 7d]

= 10d

∴ the correct option is (b)

**Question 29.**

If for A.P., T_{25} — T_{20} = 15 then d =

A. 3

B. 5

C. 20

D. 25

**Answer:**

We know that, T_{n} = a + (n – 1)d

So we have, T_{25}= a + (25 – 1)d

⇒ T_{25}= a + 24d

Now we have, T_{20}= a + (20 – 1)d

⇒ T_{20}= a + 19d

It is given that, T_{25} — T_{20} = 15

So we can say that,

⇒ 15 = [a + 24d] – [a + 19d]

⇒ 15 = 5d

⇒ d = 3

∴ the correct option is (a).