Cho \(\widehat{xOy}=120^o\) . Tia \(Oz\) nằm trong \(\widehat{xOy}\). Tia \(Om\) nằm giữa hai tia \(Ox\) và \(Oz\) sao cho : \(\widehat{xOm}=\dfrac{1}{3}\widehat{xOz}\) . Tia \(On\) nằm trong \(\widehat{zOy}\) sao cho : \(\widehat{zOn}=\dfrac{2}{3}\widehat{zOy}\) . Tính \(\widehat{mOn}\)
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