Question

Sn=77,an=20,an=-26,a1=?

arithmethic Series

Answer 1

See below

Explanation:

Assuming you meant `n=20`

`S_n= n/2(a_1+a_n)`

`S_n= 20/2(a_1-26)`

`77= 10a_1-260`

`337= 10a_1`

`a_1=33.7`