| Montrer que si {f(x,y)=F(\rho,\theta)}, alors : {\Delta(f)=\dfrac{\partial{}^2f}{\partial x^2}\!+\!\dfrac{\partial{}^2f}{\partial y^2}= \dfrac{\partial{}^2F}{\partial\rho^2} \!+\!\dfrac{1}{\rho^2}\,\dfrac{\partial{}^2F}{\partial\theta^2} \!+\!\dfrac{1}{\rho}\,\dfrac{\partial F}{\partial \rho}} |