a/ Xét \(ΔABC\) và \(ΔAHB\):
\(\widehat{A}\):chung
\(\widehat{ABC}=\widehat{AHB}(=90^\circ)\)
\(→ΔABC\backsim ΔAHB(g-g)\)
b/ Xét \(ΔBHC\) và \(ΔABC\):
\(\widehat{C}\):chung
\(\widehat{CHB}=\widehat{CBA}(=90^\circ)\)
\(→ΔHBC\backsim ΔABC(g-g)\)
\(→\dfrac{CH}{BC}=\dfrac{BC}{CA}\)
\(→BC^2=CH.CA\)
c/Sai đề