(a) Verify that u(x, t) = v(x + ct) + w(x - ct) with any twice differentiable functions v and w satisfies (1).

(b) Verify that u = 1/√x² + y² + z² satisfies (6) and u = ln (x² + y²) satisfies (3). Is u = 1/√x² + y² a solution of (3)? Of what Poisson equation?