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Math Help - Summation

  1. #1
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    Summation

    My textbook states.

    -2\sum_{i=k-4}^{2}i=-(7-k)(k-2)

    But I got the summation to equal -2k+4. I don't see how the textbook got the LHS to equal to the RHS. Thanks in advance.
    Last edited by Simplicity; November 23rd 2009 at 02:58 AM.
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  2. #2
    MHF Contributor alexmahone's Avatar
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    Quote Originally Posted by Simplicity View Post
    My textbook states.

    -2\sum_{i=k-4}^{2}i=-(7-k)(k-2)

    But I got the summation to equal -2k+4. I don't see how the textbook got the LHS to equal to the RHS. Thanks in advance.
    S_n=\frac{n}{2}(a+l)

    n=2-(k-4)+1=7-k

    a=k-4

    l=2

    -2\sum_{i=k-4}^{2}i=-2(\frac{7-k}{2})(k-4+2)

    =-(7-k)(k-2)
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