Update cheatsheet.fr.md

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

@@ -62,7 +62,13 @@ $`U(x,t) =A\;\big[cos(\underbrace{kx - \omega t}_{\text{  posons }\\ kx - \omega
 
 $`\begin{align} U(&x,t) = U_1(x,t) + U_2(x,t) \\
 &\\
-&=A\;\big[cos(\underbrace{kx - \omega t}_{\text{  posons }\\ kx - \omega t \,=\, \alpha} + \varphi_1) + cos(\underbrace{kx - \omega t}_{=\; \alpha} + \varphi_1)\,\big]
+&=A\;\big[\,cos(\underbrace{kx - \omega t}_{\text{  posons }\\ kx - \omega t \,=\, \alpha} + \varphi_1) + cos(\underbrace{kx - \omega t}_{=\; \alpha} + \varphi_1)\,\big]
+&\\
+&=A\;\big[\,cos\Big(\alpha + \dfrac{\varphi_1+\varphi_1)}{2} + \dfrac{\varphi_2-\varphi_2)}{2}\Big)
+      + \,cos\Big(\alpha + \dfrac{\varphi_2+\varphi_2)}{2} + \dfrac{\varphi_1-\varphi_1)}{2}\Big)\,\Big]\\
+&\\
+&=A\;\big[\,cos\Big(\alpha + \dfrac{\varphi_1+\varphi_2)}{2} + \dfrac{\varphi_1-\varphi_2)}{2}\Big)
+      + \,cos\Big(\alpha + \dfrac{\varphi_1+\varphi_2)}{2} - \dfrac{\varphi_1-\varphi_2)}{2}\Big)\,\Big]
      \end{align}`$