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waealu

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## Homework Statement

I am supposed to find the value of the infinite series:

[tex]\sum_{n=0}^{+\infty}\frac{\pi\cos(n)}{5^n}[/tex]

## Homework Equations

I asked this question before on this forum and

*micromass*told me that I should use cos(n)=((e^i)^n+(e^(-i))^n)/2. That equation worked and I was able to find the correct answer. I used that equation to find a "a" and an "r" in order to find the solution using =a/(1-r).

But my instructor said he would like me to try and solve it by only using real numbers.

How would I start to solve that without the imaginary numbers?

Thanks

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