Question

If nth term of an AP is (2n+1), find the sum of the first 3 terms

Answer 1

Answer:

15

Step-by-step explanation:

1st term=3

3rd term=7

sum of first 3 terms are n/2(a+l)

3/2(3+7)

3/2×10

=15

Answer 2

Answer:

since, an=2n+1

so,

a=2×1+1=3

A3=2×3+1=7

we know that,

Sn=n/2(a+an)

S3=3/2(3+a3)

=3/2(3+7)

=3/2(10)

=3×5

=15