Giải thích các bước giải:
Ta có $Ox\perp Oy\to OA\perp OB$
$\to \Delta OAB$ vuông tại $O$
$\to \widehat{OAB}+\widehat{OBA}=90^o$
$\to \widehat{mAB}+\widehat{nBA}=(180^o-\widehat{mAx}-\widehat{BAO})+(180^o-\widehat{ABO}-\widehat{nBy})$
$\to \widehat{mAB}+\widehat{nBA}=(180^o-2\widehat{BAO})+(180^o-2\widehat{ABO})$
$\to \widehat{mAB}+\widehat{nBA}=180^o-2\widehat{BAO}+180^o-2\widehat{ABO}$
$\to \widehat{mAB}+\widehat{nBA}=360^o-2(\widehat{BAO}+\widehat{ABO})$
$\to \widehat{mAB}+\widehat{nBA}=360^o-2\cdot 90^o$
$\to \widehat{mAB}+\widehat{nBA}=180^o$
$\to Am//Bn$