Update cheatsheet.fr.md

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

@@ -138,6 +138,15 @@ POUR L'ÉTUDE DES PHÉNOMÈNES DE PROPAGATION
 \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}`$**
 
+  **$`\overrightarrow{\Delta}=
+   \left(
+   \begin{array}{l}
+    \dfrac{\partial^2 U_x}{\partial x^2} + \dfrac{\partial^2 U_x}{\partial y^2} + \dfrac{\partial^2 U_x}{\partial z^2}\\
+    \dfrac{\partial^2 U_x}{\partial x^2} + \dfrac{\partial^2 U_x}{\partial y^2} + \dfrac{\partial^2 U_x}{\partial z^2}\\
+    \dfrac{\partial^2 U_x}{\partial x^2} + \dfrac{\partial^2 U_x}{\partial y^2} + \dfrac{\partial^2 U_x}{\partial z^2}
+    \end{array}
+   \right)`$**
+
   **$`\overrightarrow{\Delta}=
    \left(
    \begin{array}{l}