diff --git a/11-Extended-Kalman-Filters.ipynb b/11-Extended-Kalman-Filters.ipynb
index 68b07cf..e70d363 100644
--- a/11-Extended-Kalman-Filters.ipynb
+++ b/11-Extended-Kalman-Filters.ipynb
@@ -455,7 +455,7 @@
     "This gives us the equalities:\n",
     "\n",
     "$$\\begin{aligned}\n",
-    "\\theta &= \\tan^{-1} \\frac y x\\\\\n",
+    "\\epsilon &= \\tan^{-1} \\frac y x\\\\\n",
     "r^2 &= x^2 + y^2\n",
     "\\end{aligned}$$ "
    ]