Update cheatsheet.fr.md

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

@@ -55,6 +55,21 @@ $`\newcommand{\ddpt}[1]{\overset{\large\bullet\bullet}{#1}}`$
 
 
 * Son amplitude est :   
+  <br>
+  **$`A_{résult.} &= \left| \,2\,A\cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2} \Big) \,\right|`$**   
+  <br>
+  $`\quad\quad=\sqrt{4\,A^2 \cdot cos^2\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)}`$   
+  <br>
+  $`\color{blue}{\scriptsize{\left.\begin{align} \quad\quad &cos(a+b)=cos(a)cos(b)-sin(a)sin(b)\\
+                     &cos(a-b)=cos(a)cos(b)+-sin(a)sin(b)\end{align}
+               \right\}\Longrightarrow\\
+        \quad\quad cos^2(a)=cos(a)cos(a)=\dfrac{1}{2}[cos(a+a)+cos(a-a)]\\
+        \quad\quad\quad\quad=\dfrac{1}{2}[1 + cos(2a)]}}`$   
+   <br>
+   $`\quad\quad \color{brown}{=\sqrt{2\,A^2 \cdot \big(1 + cos\,(\varphi_1 - \varphi_2)\big)}}`$
+
+
+
   $`\begin{align} \color{brown}{A_{résult.} &= \left| \,2\,A\cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2} \Big) \,\right|}\\
   &\\
   &=\sqrt{4\,A^2 \cdot cos^2\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)}