Question

The sum of n terms of an AP is 2n2+5n.find the AP hence find the 15th term

Answer 1

Given that  sum of n terms of an AP is $2n^2+5n$

a1= S1 = 2(1)+5 = 7

S2 = a1+a2 = 2(4)+5(2) = 18

a2 = S2-S1 = 11

d = a2-a1 = 11-7 =4

Hence the AP is

7, 11, 15, 19,......

a15 = 15th term = 7+(15-1)4 = 63