Q31 of 202 Page 9

In an A.P., the sum of first n terms is . Find its 25th term.

Given, Sn=

Let an be the nth term of the A.P.


an= Sn – Sn – 1


= 3n2/2 + 13n/2 – 3(n – 1)2/2 – 13(n – 1)/2


= {n2 – (n – 1)2} + {n – (n – 1)}


= 3n - +


= 3n + = 3n + 5


Put n = 25


a25= 3(25) + 5


= 75 + 5 = 80


Therefore, 25th term is a25 = 80


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