Question

Geometric sequences.

Answer 1

Sn = (a1 . (1 - r^n))/(1 - r)

r = -5

n = 9

S9 = (a1 . (1 - (-5)^9))/(1 - (-5))

S9 = (1 . (1 + 1953125))/(1 + 5)

S9 = 1953126/6

S9 = 325521