Question

S9: {2, -4, 8, . . .} *

Answer 1

684.

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.