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Sequences and Series · Question 17

JEE Main 2026 (Online) 28th January Morning ShiftMCQMedium📈 Topic in 19% papers+4−1
1

The value of k=1 (-1)k+1 ( k(k+1)k! ) is

  1. e/2
  2. e
  3. 2/e
  4. 1/e

Answer

(4) 1/e

Solution

k(k+1)k! = k+1(k-1)!

Put m = k − 1 (m = 0, 1, 2, …): (-1)k+1 = (-1)m

S = m=0 (-1)m m + 2m!

= m=1 (-1)m(m-1)! + 2 m=0 (-1)mm!

= - j=0 (-1)jj! + 2e-1

= -e-1 + 2e-1

S = 1e

Answer: (4)

JEE Main 2026 (Online) 28th January Morning Shift

Keep practising — Sequences and Series

2

The common difference of the A.P.: a1, a2, , am is 13 more than the common difference of the A.P.: b1, b2, , bn. If b31 = -277, b43 = -385 and a78 = 327, then a1 is equal to

  1. 21
  2. 19
  3. 24
  4. 16
JEE Main 2026 (Online) 28th January Morning Shift
3

In a G.P., if the product of the first three terms is 27 and the set of all possible values for the sum of its first three terms is R - (a, b), then a2 + b2 is equal to .

JEE Main 2026 (Online) 28th January Morning Shift
4

6326 + 10 1325 + 10 2324 + 10 22323 + + 10 2243 is equal to :

  1. 226
  2. 325
  3. 326
  4. 225
JEE Main 2026 (Online) 28th January Evening Shift
5

If r=125 ( rr4 + r2 + 1 ) = pq, where p and q are positive integers such that (p, q) = 1, then p + q is equal to .

JEE Main 2026 (Online) 28th January Evening Shift
6

Let 729, 81, 9, 1, be a sequence and Pn denote the product of the first n terms of this sequence. If 2 n=140(Pn) 1n = 3 -13 and ( , ) = 1, then + is equal to

  1. 73
  2. 74
  3. 75
  4. 76
JEE Main 2026 (Online) 24th January Morning Shift
7

Consider an A.P.: a1, a2, , an; a1 > 0. If a2 - a1 = -34, an = 14a1, and i=1n ai = 5252, then i=117 ai is equal to

  1. 238
  2. 136
  3. 476
  4. 952
JEE Main 2026 (Online) 24th January Morning Shift
8

Let a1, a2, a3, a4 be an A.P. of four terms such that each term of the A.P. and its common difference l are integers. If a1 + a2 + a3 + a4 = 48 and a1a2a3a4 + l4 = 361, then the largest term of the A.P. is equal to

  1. 27
  2. 24
  3. 23
  4. 21
JEE Main 2026 (Online) 24th January Evening Shift
9

( 13 + 47 ) + ( 132 + 13 47 + 4272 ) + ( 133 + 132 47 + 13 4272 + 4373 ) + up to infinite terms, is equal to

  1. 74
  2. 43
  3. 65
  4. 52
JEE Main 2026 (Online) 24th January Evening Shift
10

Let k=1n ak = n2 + n. If a10 = 59 and a6 = 7a1, then + is equal to :

  1. 3
  2. 5
  3. 7
  4. 12
JEE Main 2026 (Online) 23rd January Evening Shift
11

If the sum of the first four terms of an A.P. is 6 and the sum of its first six terms is 4, then the sum of its first twelve terms is

  1. -26
  2. -20
  3. -24
  4. -22
JEE Main 2026 (Online) 22nd January Morning Shift

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