a, Vi: Ot la tia phan giac cua xOy => Ot nam giua 2 tia Ox, Oy va xOt = tOy = 120* : 2 = 60*
b, Vi: Ot' la tia doi cua Ot => tOt' = 180*
Vi: tOt' = 180*, tOy = 60*
=> tOy < tOt' (60* < 180*)
=> Tia Oy nam giua 2tia con lai
=> tOy + yOt' = tOt'
Ma: tOy = 60*; tOt' = 180*
=> yOt = tOt' - tOy = 180* - 60* = 120*