Partial differentiation

[diphabra]

If u(x,y,z,t)=f(t+r)/r + g(t-r)/r and r^(2) =x^(2)+y^(2)+z^(2) prove that d^(2)u/dx^(2) +d^(2)u/dy^(2)+d^(2)u/dz^(2)=d^(2)u/dt^(2)