Question

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

(a) 6n+3 

(b) 15 

(c) 12 

(d) 21

Answer 1

Correct answer is (b) 15

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

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

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

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

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