Question

What is the sum of a 56-term arithmetic sequence where the first term is 6 and the last term is 391?

10,719
11,116
11,513
11,910

Answer 1

`S_n=(a_1+a_n)/(2)\cdot n\\\\a_1=6;\ n=56;\ a_(56)=391\\\\subtitute\\\\S_(56)=(6+391)/(2)\cdot56=(397)/(2)\cdot56=397\cdot28=11,116`

Answer 2

The sum of 56 term is 11,116

B is correct.

Explanation:

Given:

First term , a=6

Last term l=391

n=56

Formula: Sum of nth term of Arithmetic series.

`Sum=(n)/(2)(a+l)`

Substitute the value of a, l and n into formula.

`Sum=(56)/(2)(6+391)`

Sum=11,116

Hence, The sum of 56 term is 11,116