.................................

$\widehat{mOn}$ `=` $\widehat{nOc}$ `+` $\widehat{mOc}$

$\widehat{nOc}$ `=` $\dfrac{1}{2}$$\widehat{aOc}$ `=` $\dfrac{1}{2}$ `.` $40^{o}$ `=` $20^{o}$

$\widehat{mOc}$ `=` $\widehat{mOa}$ `-` $\widehat{aOc}$ `=` $\dfrac{1}{2}$$\widehat{aOb}$ `-` $40^{o}$ = $\dfrac{1}{2}$ `.` $120^{o}$ `-` $40^{o}$ `=` $60^{o}$ `-` $40^{o}$ `=` $20^{o}$

`⇒` $\widehat{mOn}$ `=` $\widehat{nOc}$ `+` $\widehat{mOc}$ `=` $20^{o}$ `+` $20^{o}$ `=` $40^{o}$

$\textit{#TranDucQuangNBK}$