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Homework Help: Series Question

  1. Sep 21, 2007 #1
    1. The problem statement, all variables and given/known data

    Hello, I am wondering if someone can help with the following. I am supposed to show that

    Series from n = 1 to inf r^n*cos(nx) = rcosx -r^2/(1-2*rcosx + r^2). I am supposed to relate it to the fact that z = rexp(ix). I know that this expression is the real part of z^n or r^n*exp(inx). But I'm not sure what to do after that?

    Any hints would be greatly welcome! Thanks!!!

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 21, 2007 #2


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    Science Advisor
    Homework Helper

    Written in exponential form it's a geometric series. Sum it.
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