Chapter 6.2, Problem 67E

$\text{Consider }f\left(x,y\right)=\text{In }\left(x^2+y^2\right)$. Show that f is a solution of the partial differential equation

$\frac{{\partial }^{2}f}{\partial x^2}+\frac{{\partial }^{2}f}{\partial y^2}=0$.