Update cheatsheet.fr.md

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

@@ -515,7 +515,17 @@ $`\forall \alpha \in \mathbb{R}\;,\,`$ **$`\large{exp{\,\alpha} = cos \,\alpha +
   \\
   &= \sqrt{A^2\,e^0}=\sqrt{A^2} = \vert\,A\,\vert^2\\
   \\
-  &\large{\mathbf{\color{brown}{= A^2}}}}\end{array}`$
+  \large{\mathbf{\color{brown}{&= A^2}}}\end{array}`$
+
+  <br>
+  $`\begin{array}
+      \large{\mathbf{\color{brown}{\quad \sqrt{\underline{A}\,\underline{A}^{\ast}}}}} &= \sqrt{A\,e^{\,i\,\varphi}\times A\,e^{\,-i\,\varphi}}\\  
+  \\
+  &= \sqrt{A^2\,e^{\,i\,\varphi}\,e^{\,-i\,\varphi}} = \sqrt{A^2\,e^{\,(i-i)\,\varphi}}\\
+  \\
+  &= \sqrt{A^2\,e^0}=\sqrt{A^2} = \vert\,A\,\vert^2\\
+  \\
+  &\large{\mathbf{\color{brown}{= A^2}}}\end{array}`$