diff --git a/quad9/electrodinamica/resum/tema1.tex b/quad9/electrodinamica/resum/tema1.tex
index ff36dc8..344c418 100644
--- a/quad9/electrodinamica/resum/tema1.tex
+++ b/quad9/electrodinamica/resum/tema1.tex
@@ -349,7 +349,7 @@
 \end{defi}
 
 \begin{prop}
-  $P \cdot P \equiv P^2$ és invariant.
+  $P \cdot P \equiv P^2 = m^2 c^2$ és invariant.
 \end{prop}
 
 \begin{prop}