Em tự vẽ hình nhé.
\(\begin{array}{l}
Do\,\,\overrightarrow {AC} = \dfrac{1}{2}\overrightarrow {NC} \Rightarrow \overrightarrow {NA} = \overrightarrow {AC} \\
\overrightarrow {AK} = \dfrac{1}{2}\left( {\overrightarrow {AN} + \overrightarrow {AM} } \right) = \dfrac{1}{2}\left( { - \overrightarrow {AC} + \dfrac{1}{2}\overrightarrow {AB} } \right)\\
= - \dfrac{1}{2}\overrightarrow {AC} + \dfrac{1}{4}\overrightarrow {AB} \\
+ )\,\overrightarrow {KD} = \overrightarrow {KA} + \overrightarrow {AD} = - \left( { - \dfrac{1}{2}\overrightarrow {AC} + \dfrac{1}{4}\overrightarrow {AB} } \right) + \dfrac{1}{2}\overrightarrow {AC} \\
= \overrightarrow {AC} - \dfrac{1}{4}\overrightarrow {AB}
\end{array}\)
