Update cheatsheet.fr.md

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

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      $`div\,\overrightarrow{U}=\dfrac{\partial U_x}{\partial x}+\dfrac{\partial U_y}{\partial y}+\dfrac{\partial U_z}{\partial z}`$
    * La combinaison des deux expressions permet d'exprimer la 
      *divergence du gradient de $`f`$ en coordonnées cartésiennes* :   
-     **$`div\,\overrightarrow{grad}\,f`$**$`\;= \dfrac{\partial}{\partial x}\big(\dfrac{\partial f}{\partial x}\big)+\dfrac{\partial}{\partial y}\big(\dfrac{\partial f}{\partial y}\big)+\dfrac{\partial}{\partial z}\big(\dfrac{\partial f}{\partial z}\big)`$
      <br>
-    
+     *$`div\,\overrightarrow{grad}\,f`$*$`\;= \dfrac{\partial}{\partial x}\left(\dfrac{\partial f}{\partial x}\right)+\dfrac{\partial}{\partial y}\left(\dfrac{\partial f}{\partial y}\right)+\dfrac{\partial}{\partial z}\left(\dfrac{\partial f}{\partial z}\right)`$
+     *$`\quad = \dfrac{\partial^2 f}{\partial x^2}+\dfrac{\partial^2 f}{\partial y^2}+\dfrac{\partial^2 f}{\partial z^2}`$*   
+     <br>
+     et l'**opérateur $`div\,\overrightarrow{grad}`$ en coordonnées cartésiennes** :   
+     <br>
+     **$`div\,\overrightarrow{grad}= \dfrac{\partial^2}{\partial x^2}+\dfrac{\partial^2}{\partial y^2}+\dfrac{\partial^2}{\partial z^2}`$*