a) Ta có:
$MB = 3MA$
$\Rightarrow AM = \dfrac{1}{4}AB$
$\Rightarrow \overrightarrow{AM} = \dfrac{1}{4}\overrightarrow{AB}$
$NC = 2NA$
$\Rightarrow AN = \dfrac{1}{3}\overrightarrow{AC}$
$\Rightarrow \overrightarrow{AN} = \dfrac{1}{3}\overrightarrow{AC}$
b) Ta có:
$\overrightarrow{MN} = \overrightarrow{MA} + \overrightarrow{AN}$
$= - \overrightarrow{AM} + \overrightarrow{AN}$
$=- \dfrac{1}{4}\overrightarrow{AB} + \dfrac{1}{3}\overrightarrow{AC}$
