Update cheatsheet.fr.md

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

@@ -1649,6 +1649,10 @@ $`\quad\quad \;\; = \cdots`$
 
    $`\quad = \mathbf{\underbrace{2\,A\,\,cos\left(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B\right)}_{
    \color{blue}{\text{terme de fréquence} \\ \left|\dfrac{\nu_1 - \nu_2}{2}\right| = \left|\dfrac{440 - 441}{2}\right|=0,5\,Hz\;\lt\;10\,Hz}}
+    \times cos\,(\omega_{moy} t + \varphi_A)}`$   
+   
+   $`\quad = \mathbf{\underbrace{2\,A\,\,cos\Big(2\pi\dfrac{\nu_1 - \nu_2}{2} t + \varphi_B\Big}_{
+   \color{blue}{\text{terme de fréquence} \\ \Big|\dfrac{\nu_1 - \nu_2}{2}\Big| = \Big|\dfrac{440 - 441}{2}\Big|=0,5\,Hz}}
    \right) \times cos\,(\omega_{moy} t + \varphi_A)}`$