Update cheatsheet.fr.md

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

@@ -1681,11 +1681,11 @@ $`= 2\,A\,\,cos\big(\Delta \omega_{1-2} t + \varphi_B) \times cos\,(\omega_{moy}
   <br>
   $`\begin{align}\quad\Longleftrightarrow & cos^2\big(\pi (\nu_1 - \nu_2) t + \varphi_B\big)\\
   &\quad\quad\quad =cos^2\big(\underbrace{\pi (\nu_1 - \nu_2) t + \varphi_B \pm \mathbf{\color{blue}{\pi}}}_{
-  \color{blue}{+\;\Longleftrightarrow \nu_1 - \nu_2\gt 0 \\ -\;\Longleftrightarrow \nu_1 - \nu_2\gt 0}}\big)\end{align}`$ 
+  \color{blue}{+\pi\;\Longleftrightarrow \;\nu_1 - \nu_2\;\gt\; 0 \\ -\pi\;\Longleftrightarrow \;\nu_1 - \nu_2\;\gt\; 0}}\big)\end{align}`$ 
   <br>
   $`\quad\Longleftrightarrow \pi\, |\,\nu_1 - \nu_2\,|\, T = \pi`$   
   <br>
-  **$`\quad\Longleftrightarrow \mathbf{T = |\,\nu_1 - \nu_2\,|}`$** 
+  **$`\quad\Longleftrightarrow \mathbf{T = \dfrac{1}{|\,\nu_1 - \nu_2\,|}}`$** 
 
 
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