$\displaystyle u=\frac{1}{r}$ and $\displaystyle r=\sqrt{x^2+y^2}$ Calculate the Laplacian L(u) and give your anser in terms of r. Use dr/dx=d/x etc.

$\displaystyle \frac{du}{dx}=\frac{du}{dr}=\frac{-1}{r^2}\frac{dr}{dx}=\frac{-x}{r^3}$

now I'm stuck calculating the second order derivative