Ta có: \(\widehat{xOA}=\frac{\widehat{AOB}}{2}\) và \(\widehat{yOA}=\frac{\widehat{AOC}}{2}\)
\(\Rightarrow\widehat{xOy}=\widehat{xOA}+\widehat{yOA}=\frac{\widehat{AOB}+\widehat{AOC}}{2}=\frac{180^o}{2}=90^o\)
Hay \(\widehat{xOy}\) là góc vuông (ĐPCM)