Update cheatsheet.fr.md

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

@@ -119,6 +119,16 @@ $`\overrightarrow{rot}\,\overrightarrow{E}=-\dfrac{\partial \overrightarrow{B}}{
 $`\Longrightarrow \overrightarrow{rot}\,\left(\overrightarrow{E}+\dfrac{\partial \overrightarrow{A}
 }{\partial t}\right)=\overrightarrow{0}`$
 
+$`\quad = A\cdot 
+\;\underbrace{cos \Big[\,\Big(\omega t - \vec{k}\cdot\vec{r}  + \varphi'\Big) - \dfrac{\pi}{2}}_{\color{blue}{cos(a-\pi/2)\\\;=cos(a)\,cos(\pi/2)+sin(a)\,sin(\pi/2)\\=\;sin(a)}}\Big]`$
+
+ $`\color{blue}{\scriptsize{\left.\begin{align} \quad\quad &cos(a+b)=cos(a)cos(b)-sin(a)sin(b)\\
+                     &cos(a-b)=cos(a)cos(b)+sin(a)sin(b)\end{align}
+               \right\}\Longrightarrow\\
+        \quad\quad cos^2(a)=cos(a)cos(a)=\dfrac{1}{2}[cos(a+a)+cos(a-a)]\\
+        \quad\quad\quad\quad=\dfrac{1}{2}[1 + cos(2a)]}}`$   
+        
+
 <br>