Update cheatsheet.fr.md

12.temporary_ins/69.waves/30.n3/20.overview/cheatsheet.fr.md

@@ -57,9 +57,10 @@ $`\newcommand{\ddpt}[1]{\overset{\large\bullet\bullet}{#1}}`$
 * Son amplitude est :   
   $`\begin{align} A_{onde} &= \left| \,2\,A\cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2} \Big) \,\right|\\
   &\\
-  &=\sqrt{2\,A\cdot \underbrace{cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)\,cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)}{toto}
+  &=\sqrt{4\,A^2 \cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)\,cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)}
   \end{align}`$
 
+\underbrace{  {toto}
 
       {\left.\begin{align} 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\}\Rightarrow\\