Solution
= Solution
The <product-to-sum formula> gives the finite <Fourier series>
$$
\boxed{f(x)=\frac12\sin4x+\frac12\sin2x}.
$$
Thus the only nonzero <Fourier coefficients> are $b_2=b_4=1/2$.
= Solution
The <product-to-sum formula> gives the finite <Fourier series>
$$
\boxed{f(x)=\frac12\sin4x+\frac12\sin2x}.
$$
Thus the only nonzero <Fourier coefficients> are $b_2=b_4=1/2$.