diff --git a/documents/by-course/math-8/course-notes/main.typ b/documents/by-course/math-8/course-notes/main.typ
index a1d45c9..53e7057 100644
--- a/documents/by-course/math-8/course-notes/main.typ
+++ b/documents/by-course/math-8/course-notes/main.typ
@@ -524,7 +524,7 @@ where $x < y$, $exists z in QQ$ such that $x < z < y$.
 First, let us take the difference between $x$ and $y$.
 
 $
-  exists a,b in ZZ \
+  exists a,b,c,d in ZZ \
   x = a / b \
   y = c / d \
   y - x = (b c - a d) / (b d)