# How to handle this type of equations?

1. Jan 23, 2010

### poweng

Hi friends. I have encountered three equations that I need to find three variable An, Bn, Cn. First two are easy to handle but third one includes multiplication of infinite series(fourier series).
First two are like:

Third equation is this:

Dm is known but in order to derive a general solution we behave as if it is not known. Therefore expressions of An, Bn, Cn may include Dm. Because I can not handle 3rd equation I cant find the soulution for An, Bn and Cn. How can I deduce an equation like first two from third equation? Can we make use of orthonality principle? Any idea? Thanks in advance.

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Last edited: Jan 23, 2010