Question

The sum of n terms of a series is (n^2+2n) for all values of n. Find the first 3 terms of the series.

Answer 1

Sn=n*2+2n
S1=1*2+2(1)
=1+2
=3=a1
S2=2*2+2(2)
=4+4
=8
S2-S1=a2
8-3=5
a2=5

d=a2-a1
=5-3
=2
a1=3
a2=5
a3=3+2(2)=7

Answer 2

Answer:

same as above

I hope this will help you to understand