diff --git a/source/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.rst b/source/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.rst
index fb4c6a3..6b651a4 100644
--- a/source/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.rst
+++ b/source/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.rst
@@ -49,7 +49,7 @@ where:
 
 .. math:: 
         
-    f_x = \frac{\partial f}{\partial x} \; ; \; f_y = \frac{\partial f}{\partial x}
+    f_x = \frac{\partial f}{\partial x} \; ; \; f_y = \frac{\partial f}{\partial y}
     
     u = \frac{dx}{dt} \; ; \; v = \frac{dy}{dt}