Question

For an arithmetic sequence where a1 = 13 and the common difference is 3, find s7.

A. 162
B. 157
C. 154
D. 151

Answer 1

sn=n/2[2a+(n-1)d]
s7 means n=7

a7=7/2[2(13)+(7-1)(3)]=7/2(44)=154

Ans:C

Answer 2

C. 154

Explanation:

Here we have given that,

First Term a₁ = 13

Common Difference d = 3

Term (n ) = 7

so, we have arithmetic sequence: 13, 16, 19, 22,... and so on

Thus Sum of first 7 terms =
`(n)/(2) [2a+(n-1)d]`

= 7÷2 [2*13 + (7 - 1)*3 ]

= 3.5 [ 26 + 21 ]

= 154