$\text{Vì}$ $\widehat{xOy}$ `+` $\widehat{yOz}$ `=` $180^0$ $\text{(kề bù)}$
$\text{Mà}$ $\widehat{O1}$ `=` `\hat{xOy}/2` `;` $\widehat{O2}$ `=` `\hat{yOz}/2`
$\Rightarrow$ $\widehat{O1}$ `+` $\widehat{O2}$ `=` `\hat{xOy}/2` `+` `\hat{yOz}/2`
$\Rightarrow$ `\hat{xOy+yOz}/2 =` `180^o/{2}` `=` $90^0$
$\text{Vậy Oa vuông góc với Ob}$