Đáp án:
Kẻ O kẻ Oz $(\in \widehat{AOB})$ sao cho $Oz//xx'$
$→\widehat{OAx}+\widehat{AOz}=180^{\circ}($ hai góc trong cùng phía $)$ $(1)$
mà $\widehat{OAx}+\widehat{AOB}+\widehat{OBy}=360^{\circ}$
$→\widehat{zOB}+\widehat{OBy}=180^{\circ}$
$→Oz//yy'$
$\to \left\{\begin{matrix}
xx'//Oz & \\
yy'//Oz &
\end{matrix}\right.$
$→xx'//yy'$