\(\widehat{xOy}+\widehat{xOy'}=180^0\)(hai góc kề bù)
\(\Leftrightarrow2\cdot\widehat{xOy'}+\widehat{xOy'}=180^0\)
\(\Leftrightarrow\widehat{xOy'}=60^0\)
\(\Leftrightarrow\widehat{xOy}=120^0\)
\(\widehat{xOy}=\widehat{x'Oy'}=120^0\)
\(\widehat{xOy'}=\widehat{x'Oy}=60^0\)