Update textbook.fr.md

12.temporary_ins/40.classical-mechanics/30.n3/07.coherence/textbook.fr.md

@@ -331,11 +331,11 @@ $`\displaystyle\begin{align}
 &=\sum_{i=1}^N \sum_{j=1}^N \overrightarrow{F}_{i\rightarrow j}\\
 \\
 &=\sum_{i=1}^N \overrightarrow{F}_{i\rightarrow i}
-+\sum_{i=2}^N \sum_{j=1}^(i-1) \overrightarrow{F}_{i\rightarrow j}
-+\sum_{j=2}^N \sum_{i=1}^(j-1) \overrightarrow{F}_{i\rightarrow j}\\
++\sum_{i=2}^N \sum_{j=1}^{(i-1)} \overrightarrow{F}_{i\rightarrow j}
++\sum_{j=2}^N \sum_{i=1}^{(j-1)} \overrightarrow{F}_{i\rightarrow j}\\
 \\
 &=\sum_{i=1}^N \overrightarrow{F}_{i\rightarrow i}
-+\sum_{i=2}^N \sum_{j=1}^(i-1) \big(\overrightarrow{F}_{i\rightarrow j}+\overrightarrow{F}_{j\rightarrow i}\big)
++\sum_{i=2}^N \sum_{j=1}^{(i-1)} \big(\underbrace{\overrightarrow{F}_{i\rightarrow j}+\overrightarrow{F}_{j\rightarrow i}}_{=\,0\,(action-réaction)}\big)
 \end{align}`$