a/ Xét \(ΔADH\) và \(ΔAHB\):
\(\widehat A:chung\)
\(\widehat{ADH}=\widehat{AHB}(=90^\circ)\)
\(→ΔADH\backsim ΔAHB(g-g)\)
b/ Xét \(ΔAEH\) và \(ΔHEC\):
\(\widehat{AEH}=\widehat{HEC}(=90^\circ)\)
\(\widehat{EAH}=\widehat{EHC}\) (cùng phụ \(\widehat{C}\) )
\(→ΔAEH\backsim ΔHEC(g-g)\)
\(→\dfrac{AE}{EH}=\dfrac{HE}{EC}\)
\(↔HE^2=AE.EC\)