Update cheatsheet.fr.md

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

@@ -861,8 +861,12 @@ $`\quad\boldsymbol{\mathbf{=\color{brown}{2\,A\cdot cos\Big(\dfrac{\varphi_1-\va
   <br>
   $`\quad =A\;\big[ \,e^{\,i\;(\omega t\,-\, kx \,+ \,\varphi_1)} + e^{\,i\;(\omega t\, - \,kx \,+ \,\varphi_2)}\,\big]`$     
   <br>
-  $`\color{blue}{\scriptsize{\quad\text{Posons }\omega t - kx \,=\, \alpha}}`$   
+  $`\color{blue}{\scriptsize{\quad\text{mettons en commun les termes qui peuvent l'être}}}`$   
   <br>
+  $`A\;\big[ \,e^{\,i\;(\omega t\,-\, kx)}\,\big(\,e^{\,i\varphi_1}\,+\,e^{\,i\varphi_1}\big)`$   
+  <br>
+  $`A\;\big[ \,e^{\,i\;(\omega t\,-\, kx)}\,\big(\,e^{\,i\left(\dfrac{\varphi1 + \varphi2}{2}+\dfrac{\varphi1-\varpji2}{2}\right)}\,+\,e^{\,i\left\left(\dfrac{\varphi1 + \varphi2}{2}-\dfrac{\varphi1-\varpji2}{2}\right)}\big)`$ 
+
   $`\quad =A\;\big[\,e^{\,i\;(\alpha\, +\, \varphi_1)} + e^{\,i\;(\alpha\, + \,\varphi_2)} \,\big]`$
   <br>
   $`\color{blue}{\scriptsize{\quad\text{Rappel : } e^{\,i\;(a+b)}\;=\;e^{\,i\,a}\times e^{\,i\,b}}}`$