`a)` Trên nửa mặt phẳng bờ chứa tia `Ox`, ta có: $\widehat{xOz}$`>`$\widehat{xOy}$ `(`$100^{o}$ `>` $40^{o}$`)`
`⇒` Tia `Oy` nằm giữa `2` tia `Ox` và `Oz.`
`⇒` $\widehat{xOy}$ `+` $\widehat{yOz}$ `=` $\widehat{xOz}$
Hay $40^{o}$ `+` $\widehat{yOz}$ `=` $100^{o}$
$\widehat{yOz}$ `=` $100^{o}$ `-` $40^{o}$
$\widehat{yOz}$ `=` $60^{o}$
Vậy $\widehat{yOz}$ `=` $60^{o}$