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Thread: Sum to infinity

  1. #1
    ebb
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    Sum to infinity

    Hi, I've been doing an assignment for university and one of the questions is $\displaystyle ((r^3)/(3^r))$ (sum from r=1 to infinity). I've searched all through my notes and on the internet but I can't find the answer anywhere! Help would be appreciated!
    ebb
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  2. #2
    Super Member girdav's Avatar
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    Re: Sum to infinity

    You can use ratio test.
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  3. #3
    ebb
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    Re: Sum to infinity

    I'm not trying to find whether or not it converges, I need to evaluate it. Unless the ratio test you mentioned is another one to the one I'm thinking of?
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  4. #4
    Super Member girdav's Avatar
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    Re: Sum to infinity

    To evaluate it, write
    $\displaystyle r^3=r^2(r-1)+r^2=r(r-2)(r-1)+2r(r-1)+r^2=r(r-1)(r-2)+2r(r-1)+r(r-1)+r$.
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  5. #5
    ebb
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    Re: Sum to infinity

    Okay, I will try that. Thank you!
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