Question

the sum of n terms of an A.P is 3n^2+5.if nth term of the series is 159,then find the value of n.

Answer 1

3n²+5 is sum of nth terms. so let kth term of the a.p. be 159 . Sum till kth term is M. So, sum till (k-1)th term = M-159
so
3(k-1)²+5=3(k)²+5-159
159=3(k²-k²-1+2k)
dividing both sides by 3
53=2k-1
54=2k
k=n=27