Question

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

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

Answer 1

The correct option is D.

Explanation:

The first term of an AP is
`a_1=13`.

The common difference of AP is 3.

The sum of nth term of an AP is

`S_n=(n)/(2)[2a+(n-1)d]`

Where, a is first term and d is common difference.

We have to find S₇.

`S_7=(7)/(2)[2(13)+(7-1)3]`

`S_7=(7)/(2)[26+18]`

`S_7=(7)/(2)(44)`

`S_7=7* 22`

`S_7=154`

Therefore the correct option is D.

Answer 2

a 1 = 13, d = 3;
S n = n/2 * ( a 1 + a n )
a 7 = a 1 + 6 d = 13 + 3 * 6 = 13 + 18 = 31
S 7 = 7/2 * ( 13 + 31 ) = 7/2 * 44 = 154
Answer:
D ) 154