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NCERT Solutions for class 11 Maths | Chapter 9 - Sequences and Series

(1) The first term of a GP is 1. The sum of the third term and fifth term is 90. The common ratio of GP is
[A] 1
[B] 2
[C] 3
[D] 4
Answer: 3
(2) If the sum of the first 2n terms of the A.P. 2, 5, 8, ….., is equal to the sum of the first n terms of the A.P. 57, 59, 61, ….., then n equals
[A] 10
[B] 12
[C] 11
[D] 13
Answer: 11

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(3) If the sum of the roots of the quadratic equation ax² + bx + c = 0 is equal to the sum of the squares of their reciprocals, then a/c, b/a, c/b are in
[A] A.P.
[B] G.P.
[C] H.P.
[D] A.G.P.
Answer: H.P.
(4) If 1/(b + c), 1/(c + a), 1/(a + b) are in AP then
[A] a, b, c are in AP
[B] a², b², c² are in AP
[C] 1/1, 1/b, 1/c are in AP
[D] None of these
Answer: a², b², c² are in AP
(5) The 35th partial sum of the arithmetic sequence with terms an = n/2 + 1
[A] 240
[B] 280
[C] 330
[D] 350
Answer: 350
(6) The sum of series 1/2! + 1/4! + 1/6! + ….. is
[A] e² – 1 / 2
[B] (e – 1)² /2 e
[C] e² – 1 / 2 e
[D] e² – 2 / e
Answer: (e – 1)² /2 e
(7) If a, b, c are in G.P., then the equations ax² + 2bx + c = 0 and dx² + 2ex + f = 0 have a common root if d/a, e/b, f/c are in
[A] AP
[B] GP
[C] HP
[D] none of these
Answer: AP
(8) The sum of n terms of the series (1/1.2) + (1/2.3) + (1/3.4) + …… is
[A] n/(n+1)
[B] 1/(n+1)
[C] 1/n
[D] None of these
Answer: n/(n+1)
(9) The sum of two numbers is 13/6 An even number of arithmetic means are being inserted between them and their sum exceeds their number by 1. Then the number of means inserted is
[A] 2
[B] 4
[C] 6
[D] 8
Answer: 6
(10) If 2/3, k, 5/8 are in AP then the value of k is
[A] 31/24
[B] 31/48
[C] 24/31
[D] 48/31
Answer: 31/48
(11) Three numbers form an increasing GP. If the middle term is doubled, then the new numbers are in Ap. The common ratio of GP is
[A] 2 + √3
[B] 2 – √3
[C] 2 ± √3
[D] None of these
Answer: 2 + √3
(12) The sum of two numbers is 13/6 An even number of arithmetic means are being inserted between them and their sum exceeds their number by 1. Then the number of means inserted is
[A] 2
[B] 4
[C] 6
[D] 8
Answer: 6
(13) If a, b, c are in AP then
[A] b = a + c
[B] 2b = a + c
[C] b² = a + c
[D] 2b² = a + c
Answer: 2b = a + c
(14) If the third term of an A.P. is 7 and its 7 th term is 2 more than three times of its third term, then the sum of its first 20 terms is
[A] 228
[B] 74
[C] 740
[D] 1090
Answer: 740
(15) The sum of n terms of the series (1/1.2) + (1/2.3) + (1/3.4) + …… is
[A] n/(n+1)
[B] 1/(n+1)
[C] 1/n
[D] None of these
Answer: n/(n+1)
(16) The third term of a geometric progression is 4. The product of the first five terms is
[A] 43
[B] 45
[C] 44
[D] none of these
Answer: 45
(17) Let Tr be the r th term of an A.P., for r = 1, 2, 3, … If for some positive integers m, n, we have Tm = 1/n and Tn = 1/m, then Tm n equals
[A] 1/m n
[B] 1/m + 1/n
[C] 1
[D] 0
Answer: 1
(18) If 1/(b + c), 1/(c + a), 1/(a + b) are in AP then
[A] a, b, c are in AP
[B] a², b², c² are in AP
[C] 1/1, 1/b, 1/c are in AP
[D] None of these
Answer: a², b², c² are in AP
(19) The sum of AP 2, 5, 8, …..up to 50 terms is
[A] 3557
[B] 3775
[C] 3757
[D] 3575
Answer: 3775
(20) If a is the A.M. of b and c and G1 and G2 are two GM between them then the sum of their cubes is
[A] abc
[B] 2abc
[C] 3abc
[D] 4abc
Answer: 2abc

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