1. The problem statement, all variables and given/known data Show that the fourier series f(x) = [tex]\sum[/tex]ansin(nx) + bncos(nx) can be written as [tex]\sum[/tex]kn(cos(nx+[tex]\vartheta[/tex])) and define kn and [tex]\vartheta[/tex] where the summation is from 0 to [tex]\infty[/tex] 2. Relevant equations sin [tex]\vartheta[/tex] = cos (90 - [tex]\vartheta[/tex]) ?? 3. The attempt at a solution Well what I originally did was replace the sin term by cos (90 - nx), put cosine in terms of complex exponentials, and then try to solve the equation, but I only got what I was given in the first place and not the solution (i.e. I went in a circle). Is there some kind of property of sin or cos I could use?