Question

the sum of first n terms of AP is 3n^2+4n. find the 25th term of this ap

Answer 1

Given Sn = 3n2 + 4n

S1 = 3×1 + 4×1 = 7.

S2 = 3×4 + 4×2 = 12 + 8 = 20.

Therfore First term a1 = 7 & Second term a2 = 20 - 7 = 13 and d = 13 - 7 = 6.

Now 25th term an = 7 + ( 25 - 1) 6

a25 = 7 + 24 x 6

a25 = 151.