geometric sum problem

Here's the problem:

1 - 2 + 2^2 - 2^3 + ... + [(-1)^n][2^n]

i'm trying to get the summation with [r^(n+1) - 1] / [r - 1]

i can't figure out what r is

Quote:

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Originally Posted by ergoSum

Here's the problem:

1 - 2 + 2^2 - 2^3 + ... + [(-1)^n][2^n]

i'm trying to get the summation with [r^(n+1) - 1] / [r - 1]

i can't figure out what r is

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This is a geometric series with r = -2. So apply your formula. (The solution IS going to be a bit messy. There's no help for that.)

-Dan