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 8fe4ed136..36bd76eeb 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
@@ -157,7 +157,8 @@ en el sentido del movimiento.<br>
 et dirigé dans le sens du mouvement.<br>
 [EN] with $`\overrightarrow{e_T}`$ the unit vector tangent to the trajectory and 
 oriented in the direction of movement.<br>
-<br>$`\overrightarrow{e_T}=\dfrac{\overrightarrow{l}}{||\overrightarrow{l}||}.
+<br>$`\overrightarrow{e_T}=\dfrac{\overrightarrow{dl}}{||\overrightarrow{dl}||}
+=\dfrac{\overrightarrow{dr}}{||\overrightarrow{dr}||}`$.
 
 <!--$`\overrightarrow{e_T}
 =\dfrac{\dfrac{\partial \overrightarrow{OM}}{\partial x}}{\left| \left|