a) $\hat{xOy}-\hat{yOz}=90^o$
⇒ $\hat{xOy}=90^o+\hat{yOz}$
⇒ $\hat{xOy}+\hat{yOz}=90^o+\hat{yOz}+\hat{yOz}=150^o$
$90^o+2.\hat{yOz}=150^o$
⇒ $2.\hat{yOz}=150^o-90^o=60^o$
⇒ $\hat{yOz}=30^o$
⇒ $\hat{xOy}=90^o+30^o=120^o$
b) Ta có:
$\hat{yOz}+\hat{xOy}+\hat{xOz'}=180^o$
$30^o+120^o+\hat{xOz'}=180^o$
⇒ $\hat{xOz'}=180^o-120^o-30^o=30^o$
$Ox$ nằm giữa $Oy,Oz'$
⇒ $\hat{xOy}+\hat{xOz'}=\hat{yOz}'$
$120^o+30^o=\hat{yOz'}$
⇒ $150^o=\hat{yOz'}$
mà $\hat{xOy}+\hat{yOz}=\hat{xOz}=150^o$
⇒ $\hat{xOz}=\hat{yOz'}=150^o$