diff --git a/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md b/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md
index fafe23f3b..3abd02884 100644
--- a/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md
+++ b/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md
@@ -256,11 +256,12 @@ $`=dx\;\overrightarrow{e_x}+dy\;\overrightarrow{e_y}+dz\;\overrightarrow{e_z}`$<
 <br>$`||\overrightarrow{dl}||=\sqrt{\overrightarrow{dl}\cdot\overrightarrow{dl}}`$
 $`=\sqrt{(dl_x\;\overrightarrow{e_x}+dl_y\;\overrightarrow{e_y}+dl_z\;\overrightarrow{e_z})\cdot
 (dl_x\;\overrightarrow{e_x}+dl_y\;\overrightarrow{e_y}+dl_z\;\overrightarrow{e_z})}`$
-$`=\sqrt{(dl_x)^2\;(\overrightarrow{e_x}\cdot\overrightarrow{e_x})+(dl_y)^2\;(\overrightarrow{e_y}\cdot\overrightarrow{e_y})
+$`=\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})}`$
 $`+(2\,dl_x\,dl_y)\,(\overrightarrow{e_x}\cdot\overrightarrow{e_y})`$
 $`+(2\,dl_x\,dl_z)\,(\overrightarrow{e_x}\cdot\overrightarrow{e_z})`$
-$`+(2\,dl_y\,dl_z)\,(\overrightarrow{e_y}\cdot\overrightarrow{e_z})`$
+$`\left.+(2\,dl_y\,dl_z)\,(\overrightarrow{e_y}\cdot\overrightarrow{e_z})\right]^{1/2}`$
 $`=\sqrt{(dl_x)^2+(dl_y)^2+(dl_z)^2}`$
 $`=\sqrt{dx^2+dy^2+dz^2}=dl`$