Question

What is the sum of an 8-term geometric series if the first term is -11, the last term is 859,375, and the common ratio is -5

Answer 1

sn = a1(1 - r^n) / (1 - r)

n = 8
a1 = -11
r = -5

now we sub
s8 = -11(1 - (-5^8) / 1 - (-5)
s8 = -11 ( 1 - 390625) / (1 + 5)
s8 = -11( - 390624) / 6
s8 = 4296864/6
s8 = 716144