Update cheatsheet.fr.md

12.temporary_ins/08.grad-div-rot/20.div/20.overview/cheatsheet.fr.md

@@ -17,6 +17,10 @@ visible: false
 **$`\mathbf{\boldsymbol{div\,\overrightarrow{X}=\dfrac{1}{\rho}\;\dfrac{\partial\,(\rho\,X_{\rho})}{\partial\,\rho}
 +\dfrac{1}{\rho}\;\dfrac{\partial\,X_{\varphi}}{\partial\,\varphi}+\dfrac{\partial\,X_{z}}{\partial\,z}}}`$**
 
+<br>**$`\mathbf{
+div\,\overrightarrow{X}}`$**$`\mathbf{\;=\dfrac{d\Phi_X}{d\tau}}`$**$`\;\mathbf{=\dfrac{\partial X_x}{\partial x}+\dfrac{\partial X_y}{\partial y}+\dfrac{\partial X_z}{\partial z}
+}`$**
+
 
 ##### Expression de la divergence en coordonnées sphériques