Question

If the sum of first 2n terms of 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

Answer 1

$S_{2n}$ = $S_n$

⇒ $\frac{2n}{2}$[2.2 + (2n-1)3] = $\frac{n}{2}$[2.57 + (n-1)2]

⇒ 5n = 55