Show that for any differentiable function g(τ), u(x, z, t) = g(t − px − ηz), where p is a constant and η = /v 2 − p2, satisfies the 2d wave equation ∂2u+∂2u= 1 ∂2u. ∂x2 ∂z2 v2 ∂t2 also show that u(x, z, t) is a plane wave traveling at a horizontal (x-direction) slowness of p.