*2.5.9. Solve Laplace's equation inside a 90° sector of a circular annulus ($a < r < b$, $0 < \theta < \pi/2$) subject to the boundary conditions
(a) $u(r, 0) = 0$, $u(r, \pi/2) = 0$, $u(a, \theta) = 0$, $u(b, \theta) = f(\theta)$
(b) $u(r, 0) = 0$, $u(r, \pi/2) = f(r)$, $u(a, \theta) = 0$, $u(b, \theta) = 0$