diff --git a/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md b/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md
index 4111a1661..50b43f3ad 100644
--- a/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md
+++ b/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md
@@ -19,6 +19,11 @@ $`\overrightarrow{rot} \;\overrightarrow{B} = \mu_0\;\overrightarrow{j} +
 
 $`\Delta \overrightarrow{X} - \dfrac{1}{v_{\phi}} \; \dfrac{\partial^2 \;\overrightarrow{X}}{\partial\; t^2}=0`$
 
+L'expression de l'opérateur $`\Delta`$ en fonction des opérateurs $`grad`$, $`div`$ et $`rot`$ est :
+
+$`\Delta`$ = $`\overrightarrow{grad} \left(div\right) - \overrightarrow{rot}\, \left(\overrightarrow{rot}\right)`$
+
+
 ### Propagation du champ électromagnétique 
 
 $`\overrightarrow{rot}\,\overrightarrow{E}=-\dfrac{\partial \overrightarrow{B}}{\partial t}`$