Update cheatsheet.fr.md

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

@@ -57,7 +57,8 @@ $`\newcommand{\ddpt}[1]{\overset{\large\bullet\bullet}{#1}}`$
      $`U_1(x,t) = A\cdot cos(kx - \omega t + \varphi_1)`$.  
      $`U_2(x,t) = A\cdot cos(kx - \omega t + \varphi_2)`$.
 
-$`U(x,t) =A\;\big[cos(\underbrace{kx - \omega t}_{\text{posons }\\ kx - \omega t = \alpha} + \varphi_1)`$
+$`U(x,t) =A\;\big[cos(\underbrace{kx - \omega t}_{\text{  posons }\\ kx - \omega t \,=\, \alpha} + \varphi_1)
+    + cos(\underbrace{kx - \omega t}_{=\; \alpha} + \varphi_1)\,\big]`$
 
 * L'onde résultante $`U = U_1 + U_2`$ :   
   $`\begin{align} U(x,t) &= U_1(x,t) + U_2(x,t) \\