Kẻ $Ox'$ là tia đối tia $Ox$, khi đó ta có $$\widehat{xOy}+ \widehat{yOx'}=180^o$$
Mà $$\widehat{xOy}+ \widehat{tOy}=180^o$$
Do đó $$\widehat{y0x'}= \widehat{tOy}$$
Cũng có $$\widehat{zOx}= \widehat{zOt}$$ do $Oz$ phân giác $\widehat{xOt}$.
Cho nên $$\widehat{tOy}+ \widehat{tOz}= \widehat{yOx'}+ \widehat{zOx}$$
Hay $$\widehat{zOy}= \widehat{yOx'}+ \widehat{zOx}$$
Mà $$\widehat{zOy}+ \widehat{yOx'}+ \widehat{zOx} = 180^o$$
$$\implies 2. \widehat{zOy}=180^o \implies \widehat{zOy}= \boxed{90^o}$$