Update cheatsheet.fr.md

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

@@ -65,7 +65,8 @@ $`\newcommand{\ddpt}[1]{\overset{\large\bullet\bullet}{#1}}`$
   &=\sqrt{4\,A^2 \cdot \underbrace{cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)\,cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)}
   _{\left.\begin{align} cos(a+b)=cos(a)cos(b)-sin(a)sin(b)\\
                      cos(a-b)=cos(a)cos(b)+-sin(a)sin(b)\end{align}\right\}\Rightarrow\\
-        cos(a)cos(a)=cos^2(a)=\frac{1}{2}[cos(a+a)+cos(a-a)]frac{1}{2}[1 + cos(2a)]}
+        cos(a)cos(a)=cos^2(a)=\frac{1}{2}[cos(a+a)+cos(a-a)]\\
+        =\frac{1}{2}[1 + cos(2a)]}
   }\end{align}`$
 
 \underbrace{  {toto}