Question

What is the sum of the geometric series in which a1 = −5, r = 3, and an = −405?

Answer 1

An = a1 * r^(n-1)

-405 = -5*3^(n-1)

3^(n-1) = 81

3^4 = 81  so n-1 = 3  and n =  4

Sn  = S4  =  -5 (1 - 3^4)   / ( 1 - 3)   =  400 / -2 = -200 Answer