- #1
DieCommie
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Homework Statement
Prove the following identities:
[tex]\sum_{n=0}^{\infty} r^n \cos(n\theta) = \frac{1-r\cos(\theta)}{1-2r\cos(\theta)+r^2} [/tex]
[tex]\sum_{n=0}^{\infty} r^n \sin(n\theta) = \frac{r\sin(\theta)}{1-2r\cos(\theta)+r^2} [/tex]
Homework Equations
[tex]\cos(n\theta) = \frac{1}{2}(e^(in\theta)+e^(-in\theta))[/tex]
The Attempt at a Solution
I get it to this point...
[tex] \frac{1}{2}( \sum_{n=0}^{\infty}(re^(i\theta))^n+\sum_{n=0}^{\infty}(re^-(i\theta))^n[/tex]
But I don't know what to do next! r could be less than or greater than one. I need to do the general case where r could be either...
Also, I am not sure that the 'relevent equation' is really relevent
Any help/tips would be greatly appreciated, Thx!
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