SOLUTION: find the sum of the first nine terms of the geometric sequence 8, -16, 32, -64, 128....

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Question 985810: find the sum of the first nine terms of the geometric sequence
8, -16, 32, -64, 128....
Answer by ikleyn(52780) About Me  (Show Source):
You can put this solution on YOUR website!
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The first term is a = 8.

The common ratio is q = -2.

The sum of the first 9 terms is

s = a.%28q%5En-1%29%2F%28q-1%29 = 8.%28%28-2%29%5E9-1%29%2F%28-2-1%29 = 8.%28-512-1%29%2F%28-3%29 = 8.%28-513%29%2F%28-3%29 = 8*171 = 1368.