find a_1 in a geometric series for which s_n = 300, r= -3 and n= 4.?

Question

a. 15
b. 15/2
c. -15
d. 1/15

Answer 1

s_4 = 300
r = -3

a1 + r * a1 + r^2 * a1 + r^3 * a1 = 300
a1 (1 - 3 + 9 -27) = 300
a1 = 300/(-20)
= -15

c