What is the sum of the geometric series in which a1 = 4, r = 2, and an = 512?
answer choices:
a) sn=1,020
b)-252
c)43,690
d)-1,020

Geometric series is a series of numbers where the ratios of successive terms are the same.  To answer this problem, the information that we need to know are the first first term, the number of terms and the ratio of any successive terms. First we get the number of terms by the equation: 
an= a1(r)^(n-1) solve for n, which results to 8. 
Then the sum is computed by the equation:
Sn = a1(((r)^n)-1)/(r-1))
The answer is then A 1,020