Update cheatsheet.fr.md

12.temporary_ins/08.grad-div-rot/70.combinaisons-of-operators/20.overview/cheatsheet.fr.md

@@ -131,6 +131,13 @@ visible: false
     \end{array}
    \right)`$**
 
+  **$`\overrightarrow{\Delta}=
+   \begin{pmatrix}
+    \dfrac{\partial^2 U_x}{\partial x^2} + \dfrac{\partial^2 U_x}{\partial y^2} + \dfrac{\partial^2 U_x}{\partial z^2}\\
+    \dfrac{\partial^2 U_y}{\partial x^2} + \dfrac{\partial^2 U_y}{\partial y^2} + \dfrac{\partial^2 U_y}{\partial z^2}\\    
+    \dfrac{\partial^2 U_z}{\partial x^2} + \dfrac{\partial^2 U_z}{\partial y^2} + \dfrac{\partial^2 U_z}{\partial z^2   
+    \end{pmatrix}`$**
+
   **$`\overrightarrow{\Delta}=
    \left(
    \begin{array}{l}