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.