$\begin{array}{l}
Buoc\,\,\,thu\,\,\,2\,\,tu\,\,duoi\,\,len\,\,\,ban\,\,nham\,\,nhe.\\
\overrightarrow {KI} = - \frac{1}{2}\overrightarrow {AB} + \frac{2}{3}\overrightarrow {AC} + \frac{1}{3}\overrightarrow {AC} + \overrightarrow {BC} \\
= - \frac{1}{2}\overrightarrow {AB} + \overrightarrow {AC} + \overrightarrow {BA} + \overrightarrow {AC} \\
= - \frac{1}{2}\overrightarrow {AB} + 2\overrightarrow {AC} - \overrightarrow {AB} \\
= - \frac{3}{2}\overrightarrow {AB} + 2\overrightarrow {AC} \\
= 3\left( { - \frac{1}{2}\overrightarrow {AB} + \frac{2}{3}\overrightarrow {AC} } \right)\\
= 3\overrightarrow {KJ} .\\
\Rightarrow K,\,\,I,\,\,J\,\,thang\,\,hang,
\end{array}$
