14. Find the solution of the following problems:
\begin{equation}
(a)
\begin{aligned}
&u_t = ku_{xx} + h, &0 < x < 1, &t > 0, &h = \text{constant}, \\
&u(x, 0) = u_0(1 - \cos \pi x), &0 \le x \le 1, & &u_0 = \text{constant}, \\
&u(0, t) = 0, & & &u(l, t) = 2u_0, &t \ge 0.
\end{aligned}
\end{equation}