# The sum of a series

## Answers and Replies

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jbunniii
Science Advisor
Homework Helper
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you better use \frac{a}{b} to make clear what fraction you want to make.
make sure to use $[/itex ] (but without the latter space before the bracket) to place the expression you want between those two. Ray Vickson Science Advisor Homework Helper Dearly Missed Serie's sum of 2/n*7^n? How do we find it's sum for n=1 to n=inf? I really do not know how to start, wolfram alpha gave me the answer, but I'm not making any sense out of it. http://www.wolframalpha.com/input/?i=n=1+to+n=inf+2/(n*7^n)+sum You do it by finding the sum $$S(x) = \sum_{n=1}^{\infty} \frac{x^n}{n}$$ and then substituting the correct value of x. RGV SammyS Staff Emeritus Science Advisor Homework Helper Gold Member Is it the same as the problem in this thread? I'm not sure because you didn't use any parentheses in your expression. https://www.physicsforums.com/showthread.php?t=637936 Yes, the wolframAlpha link confirms that it's the same problem. @Badmouton, 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}}$

$\displaystyle \sum_{n=1}^{\infty} \frac{2\left(7^{-n}\right)}{n}$

$\displaystyle \sum_{n=1}^{\infty} \frac{2\left(\frac{1}{7}\right)^{n}}{n}$