By equating the sine-cosine and exponential Fourier series for the function $f$:
$f(x) = \sum_{n = 0}^\infty f_n \cdot e^{inx} = a_0 + \sum_{n=1}^{\infty} (a_n\cos(nx) + b_n\sin(nx))$
and using Euler's formula, how can we find the explicit relation between the coefficients $f_n$ and the $a_n$, $b_n$?