Question Number 7938 by tawakalitu last updated on 24/Sep/16
$${if}\:\:{f}\left({x}\right)\:=\:{xlog}\left({x}\:+\:{r}\right)\:−\:{r}\:\:{and}\:\:{r}^{\mathrm{2}} \:=\:{x}^{\mathrm{2}} \:+\:{y}^{\mathrm{2}} \\ $$$${prove}\:{that}:\:\:{f}_{{xx}} \:+\:{f}_{{yy}} \:\:=\:\:\frac{\mathrm{1}}{{x}\:+\:{r}} \\ $$