11.5.2.10 Fourier method
Problem:
Solve the boundary-value problem for the homogeneous wave equation:
\[
\begin{array}{l}
u_{t t}=a^{2} u_{x x}+t, \quad a=1.5 \\
u(x, 0)=\frac{\partial u}{\partial t}(0, x)=u(t, 0)=u(t, l)=0 .
\end{array}
\]