Update cheatsheet.fr.md

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

@@ -60,7 +60,7 @@ $`\newcommand{\ddpt}[1]{\overset{\large\bullet\bullet}{#1}}`$
 
 
 
-**$`\mathbf{U(&x,t)}`$**$`\; = U_1(x,t) + U_2(x,t)
+**$`\mathbf{U(x,t)}`$**$`\; = U_1(x,t) + U_2(x,t)`$
 <br>
 $`\begin{align} \quad &=A\;\big[\,cos(\underbrace{kx - \omega t}_{\text{  posons }\\ kx - \omega t \,=\, \alpha} + \varphi_1) + cos(\underbrace{kx - \omega t}_{=\; \alpha} + \varphi_1)\,\big]
 &\\