TC: \(\widehat{xOy}\)và \(\widehat{zOt}\)bù nhau \(\Rightarrow\)\(\widehat{xOy}+\widehat{zOt}=180^o\)(1)
mà \(\widehat{xOy}=2\widehat{zOt}\)thay vào (1)\(\Rightarrow\)\(3\widehat{zOt}=180^o\Rightarrow\widehat{zOt}=\frac{180^o}{3}=60^o\)
\(\Rightarrow\)\(\widehat{xOy}\)\(=180^o-\widehat{zOt}=180^o-60^o=120^o\)