$\\\text{TH1:Vì $\widehat{yOt}<\widehat{xOy}(60^0<20^0)$ nên tia ot nằm giữa hai tia Ox và Oy}$ $\\=>\widehat{xOy}=\widehat{yOt}+\widehat{xOt}$ $\\=>\widehat{xOt}=\widehat{xOy}-\widehat{yOt}$ $\\\widehat{xOt}=60^0-20^0=40^0$ $\\\text{Vậy $\widehat{xOt}=40^0$}$
