Simple fourier coefficients

Simple fourier coefficients

I have a forcing function of the form:
$$\alpha sin(\frac {2 \pi t} P)$$ $$0<=x<=tv$$
where $tv$ is just a constant and I am trying to find the fourier
coefficients $f_n(t)$ where $f_x(t)=\sum f_n(t)sin(\frac {n \pi x} L)$