Update cheatsheet.fr.md

12.temporary_ins/90.electromagnetism-in-vacuum/30.maxwell-electromagnetism-potentials/cheatsheet.fr.md

@@ -135,6 +135,13 @@ $`\overrightarrow{rot}\,\overrightarrow{E}=\overrightarrow{rot}\,\left(-\dfrac{\
 <br>
 $`\overrightarrow{rot}\,\Big(\overrightarrow{E}+\dfrac{\partial \overrightarrow{A}}{\partial t}\Big) =0`$   
 <br>
+$`\color{blue}{\scriptsize{\left.\begin{align} &\forall \phi\,,\, 
+  \overrightarrow{rot}\,(\overrightarrow{grad}\,\phi)=0\\
+  &\overrightarrow{rot}\,\overrightarrow{U}=0\end{align}\right\}\Longrightarrow
+  \exists\phi'\,,\, \overrightarrow{U}=-\,\overrightarrow{grad}\,\phi'}}`$   
+<br>
+$`\exists V\,,\, \overrightarrow{E}+\dfrac{\partial \overrightarrow{A}}{\partial t}=-\,\overrightarrow{grad}\,V`$
+
 
 
 $`\Longrightarrow \overrightarrow{rot}\,\left(\overrightarrow{E}+\dfrac{\partial \overrightarrow{A}