Q4 of 178 Page 551

The sum of the first n terms of an AP is (3n2 + 6n). Find the nth term and the 15th term of this AP.

Sum of first n terms = Sn = 3n2 + 6n

Now let an be the nth term of the AP.


To find: an and a15


Then an =Sn - Sn - 1


= (3n2 + 6n) - (3(n - 1)2 + 6(n - 1))


= (3n2 + 6n) - (3n2 + 3 - 6n + 6n - 6)


= 6n + 3


Now, a15 = 6(15) + 3


= 93


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