Update cheatsheet.fr.md

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

@@ -58,7 +58,12 @@ $`\newcommand{\ddpt}[1]{\overset{\large\bullet\bullet}{#1}}`$
   $`\begin{align} A_{onde} &= \left| \,2\,A\cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2} \Big) \,\right|\\
   &\\
   &=\sqrt{4\,A^2 \cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)\,cos\Big(\dfrac{\varphi_1 - \varphi_2}{2}\Big)}
-  \end{align}`$
+  \end{align}`$   
+  <br>
+  $`\small{\quadd\quadd\left.\begin{align} cos(a+b)=cos(a)cos(b)-sin(a)sin(b)\\
+                     cos(a-b)=cos(a)cos(b)+-sin(a)sin(b)\end{align}\right\}\Rightarrow\\
+        cos^2(a)=cos(a)cos(a)=\frac{1}{2}[cos(a+a)+cos(a-a)]\\
+        =\frac{1}{2}[1 + cos(2a)]}}`$
 
   $`\begin{align} A_{onde} &= \left| \,2\,A\cdot cos\Big(\dfrac{\varphi_1 - \varphi_2}{2} \Big) \,\right|\\
   &\\