Fuente TeX:

\sum_{n=0}^{\infty} \int_0^{2L}A_n\sin(\frac{n\pi x}{L})\sin(\frac{n'\pi x}{L}) + B_n\cos(\frac{n\pi x}{L})\sin(\frac{n'\pi x}{L})dx=\int_0^{2L}f(x)\sin(\frac{n'\pi x}{L})dx.