$\overrightarrow{KD}=\overrightarrow{KM}+\overrightarrow{MB}+\overrightarrow{BD}$
$\overrightarrow{KD}=\overrightarrow{KN}+\overrightarrow{NC}+\overrightarrow{CD}$
Chú ý $\overrightarrow{KM}+\overrightarrow{KN}=\overrightarrow0;\overrightarrow{BD}+\overrightarrow{CD}=\overrightarrow0$, ta có:
$2\overrightarrow{KD}=\overrightarrow{KM}+\overrightarrow{MB}+\overrightarrow{BD}+\overrightarrow{KN}+\overrightarrow{NC}+\overrightarrow{CD}\\=(\overrightarrow{KM}+\overrightarrow{KN})+(\overrightarrow{BD}+\overrightarrow{CD})+\overrightarrow{NC}+\overrightarrow{MB}=\overrightarrow{NC}+\overrightarrow{MB}=\dfrac{2}3\overrightarrow{AC}+\dfrac12\overrightarrow{AB}$
Suy ra $\overrightarrow{KD}=\dfrac13\overrightarrow{AC}+\dfrac14\overrightarrow{AB}$
Nếu có thắc mắc bạn cứ hỏi tại đây, tụi mình sẽ hỗ trợ.