Xét \(\Delta BKH\) và \(\Delta AKC\) có:
\(\left\{{}\begin{matrix}\widehat{HKB}=\widehat{CKA}\left(=90^o\right)\\\widehat{HBK}=\widehat{CAK}\left(=90^o-\widehat{ACB}\right)\end{matrix}\right.\)
\(\Rightarrow\Delta BKH\sim\Delta AKC\left(g.g\right)\text{}\)
\(\Rightarrow\frac{KH}{BK}=\frac{KC}{AK}\Rightarrow KH.KA=KB.KC\le\frac{\left(KB+KC\right)^2}{4}=\frac{BC^2}{4}\)