Question

Find S8 for the series -2 + -10 + -50 + -250 +…

Answer 1

sn = a1(1 - r^n) / (1 - r)
a1 = -2
r = 5
n = 8

s8 = -2(1 - 5^8) / (1 - 5)
s8 = -2( 1 - 390625) / -4
s8 = -2(- 390624) / -4
s8 = 781248/-4
s8 = - 195312