diff --git a/slides/chapter-09.en.md b/slides/chapter-09.en.md
index f45a49b..1456986 100644
--- a/slides/chapter-09.en.md
+++ b/slides/chapter-09.en.md
@@ -14,7 +14,7 @@ $2 \sqrt{\frac{\pi^2}{3} \cdot c_2}$
 ```
 
 Mathematical formulas are only accepted in the so-called **math mode.**
-In-line formulas must therefore be guarded by two dollar signs.
+In-line formulas must therefor be guarded by two dollar signs.
 We can also use the `equation` block environment:
 
 ``` {.lang-tex .hljs}