Tự thêm vectơ nhá!
[tex]AK=\frac{1}{2}\left ( AB+AC \right ); \\ BM=\frac{1}{2}\left ( BA+BC \right )=-AB+\frac{1}{2}AC[/tex]
Đặt [tex]AB=xAK+yBM \\ \Leftrightarrow AB=x.\frac{1}{2}(AB+AC)+y(-AB+\frac{1}{2}AC) \\ \Leftrightarrow AB=\left ( \frac{x}{2}-y \right )AB+\frac{x+y}{2}AC \\ \Leftrightarrow \left\{\begin{matrix} \frac{x}{2}-y=1\\ \frac{x+y}{2}=0 \end{matrix}\right.[/tex]
Tự giải nốt nhé!