Ta có:
Oa là phân giác \(\widehat{xOm}\)
=> \(\widehat{xOa}\) = \(\widehat{aOm}\)
=> 2 \(\widehat{aOm}\) = \(\widehat{xOm}\) (1)
Ob là phân giác \(\widehat{yOm}\)
=> \(\widehat{yOb}\) \(\widehat{aOm}\) = \(\widehat{bOm}\)
=> 2 \(\widehat{bOm}\) = \(\widehat{yOm}\) (2)
(1) + (2) =>2(\(\widehat{aOm}\) + \(\widehat{bOm}\) )= \(\widehat{xOm}\) + \(\widehat{yOm}\)
=> 2 . \(\widehat{aOb}\) = \(\widehat{xOy}\)
=> 2 \(\widehat{aOb}\) = 180\(^{\circ}\)
=> \(\widehat{aOb}\) =90 \(^{\circ}\)