Question

Evaluate the geometric series described:
a1=4 an=1024 r=-2

I've been told the answer is 684, but I have no idea how to get it!

Answer 1

divide the last term 1024 by the common ratio -2:0

1024 / -2 = -512

(-2)^ 9 = -512

so 1024 is the 9th term

Answer 2

Sum of 9 terms = 4 * (-2^9 - 1)
---------
-2 - 1