Help Solving Series Related to z = rexp(ix)

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SUMMARY

The discussion focuses on deriving the series from n = 1 to infinity for the expression r^n*cos(nx) and relating it to z = rexp(ix). The conclusion establishes that the series can be expressed as rcos(x) - r^2/(1 - 2rcos(x) + r^2). The key insight is recognizing that the series is a geometric series when written in exponential form, allowing for straightforward summation.

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Homework Statement



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!

Homework Equations





The Attempt at a Solution

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

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