we are supposed to find the Fourier Cosine series for
f(x) = 1 for 0<=x<a
f(x) = 0 for a<x<=Pi
where a is a constant
the f.s turns out to be
f(x) = a/Pi + sum over r of [(2/(Pi*r)*sin(rx)]
from which we are supposed to show that
0.5(Pi-a) = sina +0.5sin2a +1/3*sin3a ........
for 0<a<Pi
which is quite straightforward. However we are asked to prove this is also true for Pi<a<2Pi. this is where i get stuck. thanks for your help