Update cheatsheet.fr.md

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

@@ -1421,7 +1421,15 @@ $`\quad\quad \;\; = \cdots`$
 
 
 
+$`cos\theta_1+cos\theta_2 =`$$`\;\underbrace{cos\,(\theta_{moy}+\Delta\theta_{1-2})+cos\,(\theta_{moy}-\Delta\theta_{1-2}}
+_{\color{blue}{cos(a+b)=cos\,a \,cos\,b\;-\;sin\a\,sin\,b}}`$
 
+$`\quad\;\; = cos\,\theta_{moy}\;cos\,\Delta\theta_{1-2}\;-\;sin\,\theta_{moy}\;sin\,\Delta\theta_{1-2}`$
+
+de même
+
+$`cos\theta_1-cos\theta_2 =`$$`\;\underbrace{cos\,(\theta_{moy}+\Delta\theta_{1-2})-cos\,(\theta_{moy}-\Delta\theta_{1-2}}
+_{\color{blue}{cos(a-b)=cos\,a \,cos\,b\;+\;sin\a\,sin\,b}}`$