Question

The sum of n terms of an AP is given by sn = 5n²+n²+3n,find the nth term of AP

Answer 1

Here,

Sn=5n^2+n^2 +3n

S(n-1)=5(n-1)^2+(n-1)^2+3(n-1)

=5n^2-10n+1+n^2-2n+1+3n-3

=6n^2-9n-1

Now,

tn=Sn-S(n-1)

=5n^2+n^2+3n-6n^2-9n-1

=6n^2-6n^2-6n-1

=-6n-1

So,tn=-6n-1