Question

The sum of first n terms of an AP is given by (n^2+5n). Find its 25th term

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.

Answer 2

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