f(x) = cosax between pi and -pi and a is a constant
can find the an coefficent to be (sin(n+a)x)/(n+a) + (sin(n-a)x)/(n-a) but cant get any further.
This makes no sense. The coefficient can't be a function of x. Did you forget to evaluate at and ?
cos(ax) is an even function- it will have no "sin(nx)" terms in it. The coefficient of cos(nx) will be
for n> 0.
The n= 0 term is .
As I said above, all coefficients of "sin(nx)" will be 0.