Ta có: \(\widehat{BOC}=\dfrac{\widehat{AOB}}{2}\)
\(\widehat{BOD}=\dfrac{\widehat{BOA'}}{2}\)
Do đó: \(\widehat{BOC}+\widehat{BOD}=\dfrac{1}{2}\left(\widehat{AOB}+\widehat{BOA'}\right)\)
\(\Leftrightarrow\widehat{DOC}=\dfrac{1}{2}\cdot180^0=90^0\)
hay OC\(\perp\)OD