Update cheatsheet.fr.md

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

@@ -564,7 +564,8 @@ $`\quad\boldsymbol{\mathbf{=\color{brown}{2\,A\cdot cos\Big(\dfrac{\varphi_1-\va
   <br>
   $`\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) + i\;sin(kx - \omega t + \varphi_1)\big)\big]
+     &= \mathscr{Re}\big[A\cdot \big(cos(kx - \omega t + \varphi_1)\\
+     &\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]\\