Ta có: \(\overrightarrow{AK}=\overrightarrow{AB}+\overrightarrow{BK}\)
<=> \(\overrightarrow{AK}=\overrightarrow{AB}+\frac{2}{3}.\overrightarrow{BC}\)
Mà : \(\overrightarrow{BC}=\overrightarrow{AD}\)
=> \(\overrightarrow{AK}=\overrightarrow{AB}+\frac{2}{3}.\overrightarrow{AD}\)