Update cheatsheet.fr.md

12.temporary_ins/40.classical-mechanics/30.n3/40.point-dynamics/20.overview/cheatsheet.fr.md

@@ -302,7 +302,7 @@ $`\displaystyle\hspace{0.5cm}=\sum_{i=1}^N \sum_{j=1}^N \dfrac{d\overrightarrow{
 <br>
 $`\displaystyle\hspace{0.5cm}=\sum_{i=1}^N \sum_{j=1}^N \overrightarrow{F}_{i\rightarrow j}`$   
 <br>
-$`\displaystyle\hspace{0.5cm}=\sum_{i=1}^N \overrightarrow{F}_{i\rightarrow i}
+$`\displaystyle\hspace{0.5cm}=\underbrace{\sum_{i=1}^N}_{\color{blue}{cas où\\ i=j} \overrightarrow{F}_{i\rightarrow i}
 +\sum_{i=2}^N \sum_{j=1}^{(i-1)} \overrightarrow{F}_{i\rightarrow j}
 +\sum_{j=2}^N \sum_{i=1}^{(j-1)} \overrightarrow{F}_{i\rightarrow j}`$   
 <br>