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Math Help - equation

  1. #1
    Junior Member
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    equation

    And could you let me know what it's asking me to do? Thanks.
    Attached Thumbnails Attached Thumbnails equation-equation.jpg  
    Last edited by MathGuru; October 28th 2005 at 09:42 AM.
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  2. #2
    Junior Member
    Joined
    Oct 2005
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    Σ means "the sum of"
    In this case it means "the sum from when r=1 to when r=20"
    So you set r=1 first and add it to what you get when r=2... etc... up to r=20.

    Doing this by hand would take a long time so there are some tricks.

    It turns out that it is just the same as:

    Σ 5 - Σ 2r = Σ 5 + 2Σ r

    (The r=1 and 20 should still be there, just I cant put them in on a post.)

    Well 5 stays the same no matter what r is, so Σ 5 = 5x20 = 100
    Σr is just the sum of all the numbers from 1 to 20, which I believe = .5 x 20 x 21 = 210.

    Therefore your answer should be 100 + 420 = 520.
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