Question

Sms term of an ap is 2n + 1 find the sum of its first 3 terms

Answer 1

let n=1,then2×1+1=3=a
then n=2,then2×2+1=5=a2
then d=a2-a1=2
then sn=n/2{2a+(n+1)d}
s3=3/2{4+10}
s3=3/2×14
s3=7×3=21