Update cheatsheet.fr.md

10.temporary-m3p2/16.waves/20.n2/10.concept-of-wave-and-wave-phenomena-2/20.overview/cheatsheet.fr.md

@@ -905,13 +905,12 @@ _l'interférence entre ces deux ondes est destructive_
  <br>
  Tu obtiens alors :   
  <br>
- **$`\boldsymbol{\mathbf{ U(x,t)`$** =
+ **$`\mathbf{ U(x,t)}`$**
  $`\begin{align} 
-\quad = A\;\big[\,cos\Big(\alpha_{moyen} + \dfrac{\Delta\varphi}{2}\Big) \\
-&\quad\quad\quad\quad + \,cos\Big(\alpha_{moyen} - \dfrac{\Delta\varphi}{2}\Big)\,\Big]\\
+\quad & = A\;\left[\,cos\Big(\alpha_{moyen} + \dfrac{\Delta\varphi}{2}\Big) + \,cos\Big(\alpha_{moyen} - \dfrac{\Delta\varphi}{2}\Big)\,\right]\\
 &\\
-&=A\;\big[\,\underbrace{cos(\alpha)\,cos\Big(\dfrac{\Delta\varphi}{2}\Big)\,-\,sin(\alpha)\,sin\Big(\dfrac{\Delta\varphi}{2}\Big)}_{\color{blue}{\text{car   }cos(a+b)\;=\;cos\,a\,cos\,b\;-\;sin\,a\,sin\,b}}\big)\\
-&\quad + \underbrace{cos(\alpha ')\,cos\Big(\dfrac{\varphi_1-\varphi_2}{2}\Big)\,+\,sin(\alpha ')\,sin\Big(\dfrac{\varphi_1-\varphi_2}{2}}_{\color{blue}{\text{car   }cos(a-b)\;=\;cos\,a\,cos\,b\;+\;sin\,a\,sin\,b}}\big)\,\Big]\\
+&=A\;\left[\,\underbrace{cos(\alpha)\,cos\Big(\dfrac{\Delta\varphi}{2}\Big)\,-\,sin(\alpha)\,sin\Big(\dfrac{\Delta\varphi}{2}\Big)}_{\color{blue}{\text{car   }cos(a+b)\;=\;cos\,a\,cos\,b\;-\;sin\,a\,sin\,b}}\big)\\
+&\quad + \underbrace{cos(\alpha ')\,cos\Big(\dfrac{\varphi_1-\varphi_2}{2}\Big)\,+\,sin(\alpha ')\,sin\Big(\dfrac{\varphi_1-\varphi_2}{2}}_{\color{blue}{\text{car   }cos(a-b)\;=\;cos\,a\,cos\,b\;+\;sin\,a\,sin\,b}}\big)\,\right]\\
 &\\
 &=2\,A\cdot cos(\alpha ')\,cos\Big(\dfrac{\varphi_1-\varphi_2}{2}\Big)
 \end{align}`$