Gọi Oz là tia phân giác của \(\widehat{BOy}\). Ta có:
\(\widehat{AOy}+\widehat{BOy}=\widehat{AOB}+2\widehat{BOy}=2\widehat{xOB}+4\widehat{BOz}=2\widehat{xOB}+2\widehat{BOy}=2\widehat{xOy}\)
Khi đó \(\widehat{xOy}=\dfrac{\widehat{AOy}+\widehat{BOy}}{2}\)