The sum formula for a geometric sequaence is:
sum(n) = a1(1-r^n)/1-r
Where n is the number of terms, a1 is the first term and r is the common ratio:
s(8) = 3(1-1/2^8) / (1-1/2)
s(8) = 3(1-1/256) / 1/2
s(8) = 2 253/256 / 1/2
s(8) = 765/128
The answer is C.
