Do$\widehat{xOy}$và $\widehat{yOz}$ là hai góc phụ nhau
`=>`$\widehat{xOy}+$$\widehat{yOz}=$`90^0`
Mà $\widehat{xOy}$=$2.\widehat{yOz}$
`=>`$2.\widehat{yOz}+$$\widehat{yOz}=$`90^0`
`=>`$3.\widehat{yOz}=$`90^0`
`=>`$\widehat{yOz}=$`30^0`
`=>`$\widehat{xOy}$=$2.\widehat{yOz}=$`30^0 .2=60^0`