Ta có: \(\widehat{xOy}=\widehat{x'Oy'}\)( 2 góc đối đỉnh )
Mà: \(\widehat{xOy}=60^o\)
\(\Rightarrow\widehat{x'Oy'}=60^o\)
Lại có: \(\widehat{xOy}+\widehat{x'Oy}=180^o\)( 2 góc kề bù )
\(\Rightarrow60^o+\widehat{x'Oy}=180^o\)
\(\Rightarrow\widehat{x'Oy}=120^o\)
Mà \(\widehat{x'Oy}=\widehat{xOy'}\)( 2 góc đối đỉnh )
\(\Rightarrow\widehat{xOy'}=120^o\)