\(\widehat{AOC}=\widehat{BOD}\) ( 2 góc đối đỉnh )
⇒\(\frac{1}{2}\widehat{AOC}=\frac{1}{2}\widehat{BOD}\Rightarrow\widehat{COM}=\widehat{DON}\)
⇒\(\widehat{COM}+\widehat{AOM}+\widehat{AOD}=\widehat{DON}+\widehat{AOM}+\widehat{AOD}\)
⇒ \(\widehat{MON}=\widehat{COD}=180^o\)
=> M,O,N thẳng hàng hay OM và ON là haia tia đối nhau