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**mathematicalbagpiper** Let $\displaystyle u$ be the solution of the Initial Value Problem:

$\displaystyle \Delta u-u_{tt}=0$

$\displaystyle u(x,0)=\phi (r)$

$\displaystyle u_t(x,0)=\psi (r)$

Where $\displaystyle \phi$ and $\displaystyle \psi$ are continuous. Show that the solution can be represented in the region $\displaystyle {r>0, t>0}$ as:

$\displaystyle u(r,t)=\frac{1}{2r}[(r+t)\phi(r+t)+(r-t)\psi(r-t)+\int_{r-t}^{r+t}\tau\psi(\tau)d\tau]$.

I honestly have no idea where to start. :-(