$\text{a) Có: Oz nằm giữa Ox và Oy (gt)}$
$\text{⇒ $\widehat{xOz}$ + $\widehat{yOz}$ = $\widehat{xOy}$}$
$\text{⇒ $\widehat{xOz}$ = $\widehat{xOy}$ - $\widehat{yOz}$}$
$\text{⇒ $\widehat{xOz}$ = $80^{o}$ - $30^{o}$}$
$\text{⇒ $\widehat{xOz}$ = $50^{o}$}$
$\text{b) $\widehat{xOy}$ + $\widehat{yOm}$ = $\widehat{xOm}$}$
$\text{⇒ $\widehat{yOm}$ = $\widehat{xOm}$ - $\widehat{xOy}$}$
$\text{⇒ $\widehat{yOm}$ = $180^{o}$ - $80^{o}$}$
$\text{⇒ $\widehat{yOm}$ = $100^{o}$}$
$\text{c) Có: On là tia p/g của $\widehat{yOm}$ (gt)}$
$\text{⇒ $\widehat{yOn}$ = $\widehat{nOm}$ = $\dfrac{\widehat{yOm}}{2}$ = $\dfrac{100^o}{2}$ = $50^{o}$}$
$\text{Có: $\widehat{xOn}$ = $\widehat{xOy}$ + $\widehat{yOn}$}$
$\text{⇒ $\widehat{xOn}$ = $80^{o}$ + $50^{o}$}$
$\text{⇒ $\widehat{xOn}$ = $130^{o}$}$
