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Math Help - geometric sum problem

  1. #1
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    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
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by ergoSum View Post
    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

    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
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