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Math Help - Help me evaluate this series, please.

  1. #1
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    Help me evaluate this series, please.

    \sum_{r=1}^{\infty}\frac{r^3+(r+1)^3}{(r^4+r^2+1).  (r^2+r)}=
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  2. #2
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    Re: Help me evaluate this series, please.

    Smokesalot

    the general term tends to zero as n goes to infinity this means that the series might converge.
    try to use one of the known tests and verify if it converges or not.

    check here for convergence tests Convergence tests - Wikipedia, the free encyclopedia

    MINOAS
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  3. #3
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    Re: Help me evaluate this series, please.

    It does converge. It converges to ~ 1.9 but how can I show that properly?
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  4. #4
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    Re: Help me evaluate this series, please.

    Hello, smokesalot!

    I don't have the sum yet, but I've done a lot of simplifying.


    \displaystyle\sum_{r=1}^{\infty}\frac{r^3+(r+1)^3}  {(r^4+r^2+1)(r^2+r)}

    \text{Numerator:}
    . . . . . . . . \overbrace{r^3 + (r+1)^3}^{\text{sum of cubes}} \:=\:\big[r + (r+1)\big]\,\big[r^2 - r(r+1) + (r+1)^2\big] \:=\:(2r+1)(r^2+r+1)


    \text{We have: }\:\frac{(2r+1)(r^2+r+1)}{r(r+1)(r^4+r^2+1)}

    \text{Multiply by }\frac{r-1}{r-1}\!:\;\;\frac{(2r+1)(r^2+r+1)}{r(r+1)(r^4+r^2+1)}  \cdot\frac{r-1}{r-1} \;=\;\frac{(2r+1)(r^3-1)}{r(r^2-1)(r^4+r^2+1)}

    . . . . . . . =\;\frac{(2r+1)(r^3-1)}{r(r^6-1)} \;=\;\frac{(2r+1)(r^3-1)}{r(r^3-1)(r^3+1)} \;=\;\frac{2r+1}{r(r^3+1)}

    Does that help?
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  5. #5
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    Re: Help me evaluate this series, please.

    Definitely helps, thank you!
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