Question

if nth term of an ap is 2n+1 then sum of first 3 terms will be??

Answer 1

Let a1 = 2×1 +1 =3
a2 = 2×2 +1 = 5
a3 = 2×3 +1 = 7
Sn = n/2 (a+l)
S3 = 3/2 (3+7)
S3 = 3/2 × 10
S3 =: 15

Answer 2

Given 
an=2n+1
solution
for n=1
an=3=a1
for n=2
an=5=a2
for n=3
an=7
s3=a1+a2+a3
=3+5+7
=15