Như hình vẽ:
Ta có: [tex]\widehat{xOn}=\frac{1}{2}\widehat{xOy};\widehat{yOm}=\frac{1}{2}\widehat{x'Oy'}\Rightarrow \widehat{xOn}=\widehat{yOm}[/tex]
Mà: [tex]\widehat{xOn}+\widehat{nOy'}=180^{\circ}\Rightarrow \widehat{mOy'}+\widehat{nOy'}=180^{\circ}\Rightarrow ........[/tex]