Question

What is the first term in a geometric sequence if the common ratio is −2 and the sum of the first five terms is 77?

a1 = −7
a1 = 3
a1 = 7
a1= 1/7

Answer 1

Answer is a1 = 7

Using equation (derived from formula to find sum of values in a geometric series),

`77 \: = \frac{a(1 - ( { - 2}^(5)))}{1 - ( - 2)}`
Hence, solve for a.