Đáp án:
$\begin{array}{l}
9)FG = FJ\\
\Rightarrow \Delta FGJ\,\text{cân}\,F\\
\Rightarrow \widehat {FGJ} = \widehat {FJG}\\
hay\,\widehat {FGH} = \widehat {FJI}\\
\text{Xét}:\Delta FGH;\Delta FJI\\
+ FG = FJ\\
+ \widehat {FGH} = \widehat {FJI}\\
+ GH = JI\\
\Rightarrow \Delta FGH = \Delta FJI\left( {c - g - c} \right)\\
\Rightarrow HF = IF\\
10)\\
\widehat {BAC} = \widehat {BCA}\\
\Rightarrow \Delta BAC\,\text{cân}\,B\\
\Rightarrow BA = BC\\
\text{Xét}:\Delta BAE;\Delta BCD:\\
+ BA = BC\\
+ \widehat B\,chung\\
+ BD = BE\\
\Rightarrow \Delta BAE = \Delta BCD\left( {c - g - c} \right)\\
\Rightarrow \widehat {BEA} = \widehat {BDC}\\
hay\,\widehat 1 = \widehat 2
\end{array}$
