a) $\widehat{xOy}-\widehat{yOz}=90^o$
$⇒\widehat{xOy}=90^o+\widehat{yOz}$
$⇒\widehat{xOy}+\widehat{yOz}=90^o+\widehat{yOz}+\widehat{yOz}=150^o$
$⇒2.\widehat{yOz}=60^o$
$⇒\widehat{yOz}=30^o$
$⇒\widehat{xOy}=90^o+30^o=120^o$
b) Vì $\widehat{yOz}=30^o$, $\widehat{xOy}=120^o$
$⇒\widehat{yOz}<\widehat{xOy}$
Đề nếu như so sánh: $\widehat{xOz},\widehat{yOz'}$
$Oz'$ là tia đối $Oz$
$⇒\widehat{yOz}+\widehat{yOz'}=180^o$
mà $\widehat{yOz}=30^o$
$⇒\widehat{yOz'}=180^o-30^o=150^o$
Do $\widehat{xOy},\widehat{yOz}$ kề nhau
$⇒\widehat{xOy}+\widehat{yOz}=\widehat{xOz}=150^o$
Từ hai điều trên $⇒\widehat{xOz}=\widehat{yOz'}$
