Question

If nth term of an A.P. is (2n+1), what is the sum of its first three terms

Answer 1

S3=2(1)+1+2(2)+1+2(3)+1=15

Answer 2

$an = 2n + 1 \\ a1 = 2(1) + 1 = 3 \\ a2 = 5 \\ a3 = 7 \\ \\ therefore \: s3 = a1 + a2 + a3 \\ = 3 + 5 + 7 = 15 \\ \\ therefore s3 = 15$