a) Xét \(\Delta FEC,\Delta ABC\) có :
\(\left\{{}\begin{matrix}\widehat{C}:Chung\\\widehat{EFC}=\widehat{BAC}=90^o\end{matrix}\right.\)
\(\Rightarrow\Delta FEC\sim\Delta ABC\left(g.g\right)\) (1)
Xét \(\Delta FBD,\Delta ABC\) có :
\(\left\{{}\begin{matrix}\widehat{B}:chung\\\widehat{BFD}=\widehat{BAC}=90^o\end{matrix}\right.\)
\(\Rightarrow\Delta FBD\sim\Delta ABC\left(g.g\right)\) (2)
Từ (1) và (2) => \(\Delta FEC\sim\Delta FBD\left(\sim\Delta ABC\right)\)
b) Xét \(\Delta AED,\Delta HAC\) có :
\(\left\{{}\begin{matrix}\widehat{EAD}=\widehat{AHC}=90^o\\\widehat{ADE}=\widehat{HCA}\left(\Delta FEC\sim\Delta FBD\right)\end{matrix}\right.\)
\(\Rightarrow\Delta AED\sim\Delta HAC\left(g.g\right)\)