1. The problem statement, all variables and given/known data Hi, so im doing some past exam papers and there was this question; 2. Relevant equations 3. The attempt at a solution a0 and an both are equal to zero, this leaves only bn. Since you can only use the sine series for an odd function, and cos(t) is even, does this mean i have to find the odd half expansion of cos(t) with L=π ? This would be f(t) = -cos(t) for [-π,0] and cos(t) for [0,π]. Or do I use L= π/2 and do the integral from 0 to π for bn? Letting n=1 since it is the first partial sum we are talking about (can i do this?)then using the first method I get 16/(3π) and if i try the second method I get 8/(3π). But the answer is listed as 0. Any help would be great thanks.