diff --git a/book/numerical_methods/8-boundary-value-problem.ipynb b/book/numerical_methods/8-boundary-value-problem.ipynb
index e1b66351dd408aa79a02029e993f91d0ee5a2847..caab199a8d6f92e88eba3bf1ee35eb9b1967dac2 100644
--- a/book/numerical_methods/8-boundary-value-problem.ipynb
+++ b/book/numerical_methods/8-boundary-value-problem.ipynb
@@ -411,7 +411,7 @@
     "\n",
     "\n",
     "$$\n",
-    "\\text{Backward Difference: } f'= \\frac{f(x_i)-f(x_{-1})}{\\Delta x} + \\mathcal{O}(\\Delta x)\n",
+    "\\text{Backward Difference: } f'= \\frac{f(x_i)-f(x_{i-1})}{\\Delta x} + \\mathcal{O}(\\Delta x)\n",
     "$$\n",
     "\n",
     "$$\n",