Update cheatsheet.fr.md

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

@@ -1081,10 +1081,24 @@ commenter
 
 ![](sum-two-plane-wave-progressive-monochrome-complex-representation-2_L1200.gif)
 
-calculer
 
 ![](sum-two-plane-wave-progressive-monochrome-complex-representation-3_L1200.jpg)
 
+$\mathbf{A = \|\overrightarrow{U}\| =
+\sqrt{\overrightarrow{U}\cdot\overrightarrow{U}}
+=
+ \sqrt{\big(\overrightarrow{U}_1 + \overrightarrow{U}_2\big)\cdot\big(\overrightarrow{U}_1 + \overrightarrow{U}_2\big)}}$
+
+$\mathbf{ =
+ \sqrt{\overrightarrow{U}_1\cdot\overrightarrow{U}_1 + \overrightarrow{U}_2\cdot\overrightarrow{U}_2+2\;\overrightarrow{U}_1\cdot\overrightarrow{U}_2}}$
+
+$\mathbf{ =
+ \sqrt{\|\overrightarrow{U}_1\|^2+ \|\overrightarrow{U}_2\|^2+2\;\|\overrightarrow{U}_1\|\;\|\overrightarrow{U}_2\|\;cos\Big(\widehat{\overrightarrow{U}_1,\overrightarrow{U}_2}\Big)}}$
+
+$\mathbf{ =
+ \sqrt{A_1^2\;+\;A_2^2\;+\;
+2 \;A_1\,A_2\;cos(\varphi_2-\varphi_1)}}$
+
 <br>
 
 --------------------------------------------