diff --git a/chapter07.tex b/chapter07.tex
index 70fd873..120513d 100644
--- a/chapter07.tex
+++ b/chapter07.tex
@@ -1039,7 +1039,7 @@ As a final note, there is also a surprising direct formula
 for calculating the number of tilings\footnote{Surprisingly,
 this formula was discovered in 1961 by two research teams \cite{kas61,tem61}
 that worked independently.}:
-\[ \prod_{a=1}^{\lceil n/2 \rceil} \prod_{b=1}^{\lceil m/2 \rceil} 4 \cdot (\cos^2 \frac{\pi a}{n + 1} + \cos^2 \frac{\pi b}{m+1})\]
+\[ \prod_{a=1}^{\lceil n/2 \rceil} \prod_{b=1}^{\lceil m/2 \rceil} 4 \cdot \left(\cos^2 \frac{\pi a}{n + 1} + \cos^2 \frac{\pi b}{m+1}\right)\]
 This formula is very efficient, because it calculates
 the number of tilings in $O(nm)$ time,
 but since the answer is a product of real numbers,