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The sum of a series

  1. Sep 23, 2012 #1
  2. jcsd
  3. Sep 23, 2012 #2


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  4. Sep 23, 2012 #3
    you better use \frac{a}{b} to make clear what fraction you want to make.
    make sure to use [itex][/itex ] (but without the latter space before the bracket) to place the expression you want between those two.
  5. Sep 24, 2012 #4

    Ray Vickson

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    You do it by finding the sum
    [tex] S(x) = \sum_{n=1}^{\infty} \frac{x^n}{n}[/tex] and then substituting the correct value of x.

  6. Sep 24, 2012 #5


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    Yes, the wolframAlpha link confirms that it's the same problem.


    The expression you are summing, with the proper set of parentheses: 2/(n*7n).

    This equivalent to (2*7-n)/n also equivalent to (2*(1/7)n)/n .

    All of these are more readable using LaTeX, which allows you to include the summation symbol, Ʃ , along with the summation limits.

    [itex]\displaystyle \sum_{n=1}^{\infty} \frac{2}{n7^{n}}[/itex]

    [itex]\displaystyle \sum_{n=1}^{\infty} \frac{2\left(7^{-n}\right)}{n}[/itex]

    [itex]\displaystyle \sum_{n=1}^{\infty} \frac{2\left(\frac{1}{7}\right)^{n}}{n}[/itex]
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