Update cheatsheet.fr.md

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

@@ -594,6 +594,15 @@ $`\quad\boldsymbol{\mathbf{=\color{brown}{2\,A\cdot cos\Big(\dfrac{\varphi_1-\va
 * Calcul de l'onde complexe résultante :  
   <br>
   $`\color{brown}{\mathbf{\underline{U}(x,t)}}\; = \underline{U_1}(x,t) + \underline{U_2}(x,t)`$ 
+  <br>
+  $`\quad &=A\;\big[\,e^{\,i\;(kx - \omega t+ \varphi_1)} + e^{\,i\;(kx - \omega t+ \varphi_2)}\,\big]`$  
+  <br>
+  $`\color{blue}{\scriptsize{\text{Pour simplifier, poson }kx - \omega t \,=\, \alpha}}`$   
+  <br>
+
+
+cos(\underbrace{(kx - \omega t}_{\color{blue}{=\; \alpha}} + \varphi_1)\,\big]
+