Can someone explain this to me simply. I just plain do not get Fourier Series. I've got a question that says: show that f(x)=exp(-cx) for 0<x<pi =exp(cx) for -pi<x<0 can be written as a cosine series thats way too complicated for me to work out how to write here. I have no idea where to start. My lecture notes are of very little help, and all the websites and textbooks seem to be of no help at all. My only thought of where to start is to write e as functions of cos and sin, but there are no sin's in the answer, so I'm a little dubious about this. If anyone has a way of explaining this to dumbasses, then I'd love to hear it!