$\text{Kẻ đt x // ED}$
$\text{Ta có : $\widehat{D}$ = $\widehat{O1}$ (hai góc so le trong)}$
$\text{=> $\widehat{O1}$ = $60^{o}$}$
$\text{Ta có : $\widehat{O2}$ = $\widehat{O}$ - $\widehat{O1}$}$
$\text{ = $110^{o}$ - $60^{o}$ = $50^{o}$.}$
$\text{=> $\widehat{A}$ = $\widehat{O2}$ (= $50^{o}$)}$
$\text{ Mà $\widehat{A}$ và $\widehat{O2}$ là 2 góc so le trong}$
$\text{=> BA // x.}$
$\text{=> BA // ED.}$
