Answer:
684.
Step-by-step explanation:
-4/ 2 = -2 and 8 / -4 = -2 so:
this is a geometric sequence with a = 2 and r = -2.
Sn = sum of n terms =a * (1 - r^n) / ( 1 - r)
So when n = 9:
S9 = 2 (1 - (-2)^9) / (1 - (-2))
= 2 * 1026 / 3
= 684.
