I think this is a trick question... well sort of :P
for part (a) i get that the cosine Fourier Series for f(x) = cos(x) to be:
i hope that is ok, but its part (b) that is troubling me...
is all that happens as is that the cosine Fourier series of cos(x) goes to 0?
i am guessing i am missing some trick to this question?