Question

In an Ap the sum of first n terms is 3n square /2+13n/2.find the 25 term?

Answer 1

Sn = 3n^2/ 2 + 13n/ 2

=> n/2 [ 2a +(n-1)d] = n/2 (3n + 13)

=> 2a + (n-1)d = 3n + 13

=> 2a +(n-1) d = 16 + 3n - 3

=> 2a +(n-1)d = 16 + 3(n-1)


Comparing both sides, we get

2a = 16

=> a = 8

d = 3


T25 = a + 24 d

= 8 + 24 * 3

= 8 + 72

= 80


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