Question

Choose the correct answer in each of the following questions:

If the nth term of the AP is (2n + 1) then the sum of its first three terms is

(a) 6n + 3 (b) 15 (c) 12 (d) 21 [CBSE 2012]

Answer 1

n^th term of the AP, a_n = 2n + 1 (Given)

∴ First term, a₁ = 2 × 1 + 1 = 2 + 1 = 3

Second term, a₂ = 2 × 2 + 1 = 4 + 1 = 5

Third term, a₃ = 2 × 3 + 1 = 6 + 1 = 7

∴ Sum of the first three terms = a₁ + a₂ + a₃ = 3 + 5 + 7 = 15

Hence, the correct answer is option B.