SOLUTION: hi there. i need help with this problem: prove the sum of the first n term of a geometric sequence ...

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Question 438222: hi there.
i need help with this problem: prove the sum of the first n term of a geometric sequence ...
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
S+=+a+%2B+ar+%2B+ar%5E2++...+ar%5E%28n-2%29+%2B+ar%5E%28n-1%29
=a(1+%2B+r+%2B+r%5E2++...+r%5E%28n-2%29+%2B+r%5E%28n-1%29)
=a(1+%2B+r+%2B+r%5E2++...+r%5E%28n-2%29+%2B+r%5E%28n-1%29)*%28%281-r%29%2F%281-r%29%29
=a*%281-r%5En%29%2F%281-r%29
This formula is valid only when |r| < 1.