Find the sum of the geometric sequence - 3, 15, - 75, 375, ... when there are 8 terms

First, we need to solve for the common ratio from the data given by using the equation.

a (n) = a (1) r^ (n-1)

15 = - 3 r^ (2-1)

-5 = r

r = - 5

Then, we can find the sum by the expression:

S (n) = a (1) (1 - r^n) / 1-r

S (8) = - 3 (1 + 5^8) / 1+5

S (8) = - 195313