$\begin{array}{l}
Goi\,M\,la\,trung\,diem\,cua\,BC.\\
Ta\,co:2\overrightarrow {IA} + \overrightarrow {IB} + \overrightarrow {IC} = \overrightarrow 0 \\
\Leftrightarrow 2\overrightarrow {IA} + 2\overrightarrow {IM} = \overrightarrow 0 \\
\Leftrightarrow 2\left( {\overrightarrow {IA} + \overrightarrow {IM} } \right) = \overrightarrow 0 \\
\Leftrightarrow \overrightarrow {IA} + \overrightarrow {IM} = \overrightarrow 0 \\
\Rightarrow I\,la\,trung\,diem\,cua\,AM
\end{array}$
