diff --git a/chapter01.tex b/chapter01.tex
index 5428966..bb2aadc 100644
--- a/chapter01.tex
+++ b/chapter01.tex
@@ -600,7 +600,7 @@ For example,
 
 This formula can be derived as follows. Let
 \[ S = a + ak + ak^2 + \cdots + b .\]
-By multiplying both sides by $x$, we get
+By multiplying both sides by $k$, we get
 \[ kS = ak + ak^2 + ak^3 + \cdots + bk,\]
 and solving the equation
 \[ kS-S = bk-a\]