Update cheatsheet.fr.md

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

@@ -306,7 +306,8 @@ $`U(x,t)=A\cdot cos\,(\omega t - kx + \phi)`$
 
 $`\; = A\cdot cos \Big(\omega t - kx  + \underbrace{ \phi + \dfrac{\pi}{2}}_{\color{blue}{=\;\phi'}} - \dfrac{\pi}{2}\Big)`$
 $`\; = A\cdot \Big[
-\;\underbrace{cos \Big(\omega t - kx  + \phi'\Big) - \dfrac{\pi}{2}}_{cos(a-\pi/2)=cos(a)\,cos(\pi/2)+sin(a)\,sin(\pi/2)=sin(a)}\Big]`$
+\;\underbrace{cos \Big(\omega t - kx  + \phi'\Big) - \dfrac{\pi}{2}}_{\color{blue}{cos(a-\pi/2)\\=cos(a)\,cos(\pi/2)+sin(a)\,sin(\pi/2)\\=sin(a)}}\Big]`$
+$`U(x,t)=A\cdot sin\,(\omega t - kx + \phi')`$
 
 
 <!--\underbrace{ \phi + \dfrac{\pi}{2}}{=\;}- \dfrac{\pi}{2})`$-->