Giải thích các bước giải:
a) Ta có:
$\begin{array}{l}
\left\{ \begin{array}{l}
\widehat Cchung\\
\widehat {CEH} = \widehat {CFA} = {90^0}
\end{array} \right.\\
\Rightarrow \Delta ECH \sim \Delta FCA\left( {g.g} \right)
\end{array}$
Lại có:
$\begin{array}{l}
\left\{ \begin{array}{l}
\widehat {BHF} = \widehat {CHE}\left( {dd} \right)\\
\widehat {BFH} = \widehat {CEH} = {90^0}
\end{array} \right.\\
\Rightarrow \Delta BFH \sim \Delta CEH\left( {g.g} \right)\\
\Rightarrow \widehat {HBF} = \widehat {HCE}\\
\Rightarrow \widehat {EBA} = \widehat {ECH}
\end{array}$
Khi đó:
$\begin{array}{l}
\left\{ \begin{array}{l}
\widehat {EBA} = \widehat {ECH}\\
\widehat {BEA} = \widehat {CEH} = {90^0}
\end{array} \right.\\
\Rightarrow \Delta EBA \sim \Delta ECH\left( {g.g} \right)\\
\Rightarrow \dfrac{{EB}}{{EC}} = \dfrac{{EA}}{{EH}}\\
\Rightarrow EB.EH = EA.EC
\end{array}$
