Update cheatsheet.fr.md

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

@@ -1674,13 +1674,16 @@ $`= 2\,A\,\,cos\big(\Delta \omega_{1-2} t + \varphi_B) \times cos\,(\omega_{moy}
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   **$`\mathbf{U(t)=U(t+T)}`$**   
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-  $`\quad\Longleftrightarrow cos^2\left(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B\right)=cos^2\left(2\pi\dfrac{\nu_1 - \nu_2}{2} (t\mathbf{\color{blue}{+T}}) + \varphi_B + \pi\right)`$   
+  $`\begin{align}\quad\Longleftrightarrow & cos^2\left(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B\right)\\
+  &\quad\quad\quad = cos^2\left(2\pi\dfrac{\nu_1 - \nu_2}{2} (t + \mathbf{\color{blue}{T}}) + \varphi_B\right)\end{align)`$   
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-  $`\quad\Longleftrightarrow cos^2\left(\pi (\nu_1 - \nu_2) t + \varphi_B\right)=cos^2\left(\pi (\nu_1 - \nu_2) t + \varphi_B + \pi\right) \mathbf{\color{blue}{+ \pi}}`$   
+  $`\begin{align}\quad\Longleftrightarrow & cos^2\big(\pi (\nu_1 - \nu_2) t + \varphi_B\big)\\
+  &\quad\quad\quad =cos^2\big(\underbrace{\pi (\nu_1 - \nu_2) t + \varphi_B \pm \mathbf{\color{blue}{\pi}}}_{
+  \color{blue}{+\;\Longleftrightarrow \nu_1 - \nu_2\gt 0 \\ -\;\Longleftrightarrow \nu_1 - \nu_2\gt 0}}\big)\end{align)`$ 
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-  $`\quad\Longleftrightarrow 2\pi\dfrac{\nu_1 - \nu_2}{2} T = \pi`$   
+  $`\quad\Longleftrightarrow 2\pi\dfrac{|\,\nu_1 - \nu_2\,|}{2} T = \pi`$   
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-  **$`\quad\Longleftrightarrow \mathbf{T = |\nu_1 - \nu_2|}`$** 
+  **$`\quad\Longleftrightarrow \mathbf{T = |\,\nu_1 - \nu_2\,|}`$** 
 
 
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