Update cheatsheet.fr.md

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

@@ -565,8 +565,7 @@ $`\quad\boldsymbol{\mathbf{=\color{brown}{2\,A\cdot cos\Big(\dfrac{\varphi_1-\va
   $`\begin{align} U_1&(x,t) = A\cdot cos(kx - \omega t + \varphi_1)\\
      &\\
      &= \mathscr{Re}\big[A\cdot \big(cos(kx - \omega t + \varphi_1)\\
-     &\quad\quad\quad + i\;sin(kx - \omega t + \varphi_1)\big)\big]\\
-     \\
+     &\quad\quad\quad\quad + i\;sin(kx - \omega t + \varphi_1)\big)\big]\\
      &\\
      &= \mathscr{Re}\big[A\cdot e^{\,i\;(kx - \omega t + \varphi_1)} \big]\\
      &\\