Question Number 212788 by MrGaster last updated on 24/Oct/24
$$ \\ $$$${Let}\:{f}\left({x}\right)\:\mathrm{There}\:\mathrm{is}\:\mathrm{a}\:\mathrm{secondorder}\:\mathrm{continuoust} \\ $$$$\mathrm{derivaive},{t}=\sqrt{{x}^{\mathrm{2}} +{y}^{\mathrm{2}} },{g}\left({x},{y}\right)={f}\left(\frac{\mathrm{1}}{{r}}\right),\mathrm{ask}\:\frac{\partial^{\mathrm{2}} {g}}{\partial{x}^{\mathrm{2}} }+\frac{\partial^{\mathrm{2}} {g}}{\partial{y}^{\mathrm{2}} }. \\ $$