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Math Help - find the sum of the arithmetic equation.

  1. #1
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    find the sum of the arithmetic equation.

    an = 5
    d = -2
    n = 12
    Last edited by mr fantastic; September 13th 2010 at 12:40 PM.
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  2. #2
    MHF Contributor Unknown008's Avatar
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    For the first n terms the formula is given by:

    S_n = \frac{n}{2}(2a + (n-1)d)

    or

    S_n = \frac{n}{2}(a+ l)

    where l is the last term that you are counting in.

    Post what you get.
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