find sn for the geometric sequence with a1=8,n=9 and r=-1?

Question

answers:
72
17
-8
8

Answer 1

a = 8, -8, 8, -8, 8, -8, 8, -8, 8, -8
n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

at n = 9, sn = 8

Answer 2

The answer is : 8. Here's how :

This bizarre "geometric sequence" is 8 - 8 + 8 - 8 +... .

Group these by pairs; there will be one "+8" left over. So it's :

(8 - 8) + (8 - 8) + (8 - 8) + (8 - 8) + 8 = 8.

It can also be summed by the standard formula, of course. That sum is

a1 (r^n - 1)/(r - 1) = 8 ([-1]^9 - 1]/(-2) = 8 (-2) / (-2) = 8.

Live long and prosper.