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?

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.