Evaluate the constants b_n for the special case \displaystyle f(x)=\sin\left(\frac{3\pi x}{L}\right). On a single graph plot the solutions u(x,t) for t=0, \ 1, \ 2, \ 3, assuming L=\pi, \ k=1

\displaystyle u(x,t)=\sum_{n=1}^{\infty}b_n\exp(\lambda_n kt)\sin\left(\frac{n\pi x}{L}\right)

The book shows to set up solving b_n like:

\displaystyle\int_0^Lf(x)\sin\left(\frac{m\pi x}{L}\right)dx=\sum_{n=1}^{\infty}b_n\int_0^L\sin\  left(\frac{n\pi x}{L}\right)\sin\left(\frac{m\pi x}{L}\right)

Now, is the f(x) in how to setup finding b_n the f(x) in the problem?

Thanks.