Ta có: \(\overrightarrow {AK} = \dfrac{1}{2}\left( {\overrightarrow {AM} + \overrightarrow {AN} } \right)\)\( = \dfrac{1}{2}\left( {\dfrac{1}{2}\overrightarrow {AB} + \dfrac{2}{3}\overrightarrow {AC} } \right)\)\( = \dfrac{1}{4}\overrightarrow {AB} + \dfrac{1}{3}\overrightarrow {AC} \)