Question

if the nth term of an AP is [2n +1],find the sum of its first n terms.

Answer 1

an=2n+1
a1=2*1+1=3
sum of n terms=n/2[first term+n th term]
=n/2(a1+an)
=n/2(3+2n+1)
=n/2(2n+4)
=n(n+2)