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Math Help - Sequence problem

  1. #1
    Junior Member
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    Sequence problem

    <br />
\sum_{k=1}^6(-1)^k4^k<br />

    The answer is: 3276

    How do you do this problem?
    -1^6=1

    4^6=4096

    4096 != 3276
    Thanks.
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  2. #2
    Super Member
    Joined
    Jun 2009
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    Quote Originally Posted by desiderius1 View Post
    <br />
\sum_{k=1}^6(-1)^k4^k<br />

    The answer is: 3276

    How do you do this problem?
    -1^6=1

    4^6=4096

    4096 != 3276
    Thanks.
    It is a GP with first term -4, common ratio r = -4 and number of terms 6.
    So sum S = a[r^n - 1]/(r-1)
    = -4(4096 - 1)/(-4-1)
    Now find the answer.
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