Update cheatsheet.fr.md

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

@@ -597,7 +597,14 @@ $`\quad\boldsymbol{\mathbf{=\color{brown}{2\,A\cdot cos\Big(\dfrac{\varphi_1-\va
   <br>
   $`\quad =A\;\big[ \,e^{\,i\;(kx - \omega t+ \varphi_1)} + e^{\,i\;(kx - \omega t+ \varphi_2)}\,\big]`$     
   <br>
-  $`\color{blue}{\scriptsize{\text{Pour simplifier, poson }kx - \omega t \,=\, \alpha}}`$   
+  $`\color{blue}{\scriptsize{\quad\text{Posons }kx - \omega t \,=\, \alpha}}`$   
+  <br>
+  $`\quad =A\;\big[ \,e^{\,i\;(\alpha + \varphi_1)} + e^{\,i\;(\alpha + \varphi_2)}\,\big]`$
+  <br>
+  $`\color{blue}{\scriptsize{\quad\text{Rappel : }e^{\,i\;(a+b)\;=\;e^{\,i\,a}\timese^{\,i\,b}`$   
+  <br>
+  $`\quad =A\;\big[ \,e^{\,i\,\alpha}\;e^{\,i\,\varphi_1}\; + \; e^{\,i\,\alpha}\;e^{\,i\,\varphi_2}\,\big]`$   
+  $`\quad =A\;e^{\,i\,\alpha}\;\big[\,e^{\,i\,\varphi_1}\; + \; e^{\,i\,\varphi_2}\,\big]`$    
   <br>