Update cheatsheet.fr.md

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

@@ -304,8 +304,10 @@ RÉSUMÉ
 $`U(x,t)=A\cdot cos (\omega t - kx + \phi)`$
 
 
-$`\; = A\cdot cos \Big(\omega t  - kx  + \underbrace{ \phi + \dfrac{\pi}{2}}{=\;\phi'} - \dfrac{\pi}{2}\Big)`$
-$`\; = A\cdot \Big(\;\Big(cos \omega t  + \dfrac{\pi- kx  }{2} + \phi'\Big) - \dfrac{\pi}{2}\Big]`$
+$`\; = A\cdot cos \Big(\omega t  - kx  + \underbrace{ \phi + \dfrac{\pi}{2}}_{\color{blue}{=\;\phi'}} - \dfrac{\pi}{2}\Big)`$
+$`\; = A\cdot \Big[\;\underbrace{cos \Big(\omega t - kx  + \phi'\Big) - \dfrac{\pi}{2}}_{cos(a-\pi/2})=cos(a)\,cos(\pi/2)+sin(a)\,sin(\pi/2)=sin(a)}\Big]`$
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+
 <!--\underbrace{ \phi + \dfrac{\pi}{2}}{=\;}- \dfrac{\pi}{2})`$-->