Update textbook.fr.md

00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md

@@ -254,11 +254,12 @@ $`=dx\;\overrightarrow{e_x}+dy\;\overrightarrow{e_y}+dz\;\overrightarrow{e_z}`$<
 [EN] y its norm (or length) is thescalar line element :<br>
 <br>$`||\overrightarrow{dl}||=\sqrt{dl_x^2+dl_y^2+dl_z^2}=\sqrt{dx^2+dy^2+dz^2}`$<br>
 <br>$`||\overrightarrow{dl}||=\sqrt{\overrightarrow{dl}\cdot\overrightarrow{dl}}`$
-$`=\sqrt{(dl_x\;\overrightarrow{e_x}+dl_y\;\overrightarrow{e_y}+dl_z\;\overrightarrow{e_z})\cdot
-(dl_x\;\overrightarrow{e_x}+dl_y\;\overrightarrow{e_y}+dl_z\;\overrightarrow{e_z})}`$
+$`=\left[(dl_x\;\overrightarrow{e_x}+dl_y\;\overrightarrow{e_y}+dl_z\;\overrightarrow{e_z})\cdot
+(dl_x\;\overrightarrow{e_x}\rigth.`$
+$`\left.+dl_y\;\overrightarrow{e_y}+dl_z\;\overrightarrow{e_z})\right]^{1/2}`$
 $`=\left[(dl_x)^2\;(\overrightarrow{e_x}\cdot\overrightarrow{e_x})\right.`$
 $`+(dl_y)^2\;(\overrightarrow{e_y}\cdot\overrightarrow{e_y})`$
-$`+(dl_z)^2\;(\overrightarrow{e_z}\cdot\overrightarrow{e_z})}`$
+$`+(dl_z)^2\;(\overrightarrow{e_z}\cdot\overrightarrow{e_z})`$
 $`+(2\,dl_x\,dl_y)\,(\overrightarrow{e_x}\cdot\overrightarrow{e_y})`$
 $`+(2\,dl_x\,dl_z)\,(\overrightarrow{e_x}\cdot\overrightarrow{e_z})`$
 $`\left.+(2\,dl_y\,dl_z)\,(\overrightarrow{e_y}\cdot\overrightarrow{e_z})\right]^{1/2}`$