$\begin{array}{l}
17/\\
\overrightarrow {AG} = \frac{2}{3}\overrightarrow {AM} \Rightarrow A\,sai.\\
18/\\
M\,la\,TD\,cua\,BC \Leftrightarrow \overrightarrow {MB} + \overrightarrow {MC} = \overrightarrow 0 \Leftrightarrow \overrightarrow {MB} = - \overrightarrow {MC} \\
19/\\
A.\overrightarrow {AB} = 2\overrightarrow {AM} \left( {dung} \right)\\
B.\overrightarrow {AC} = 2\overrightarrow {NC} \left( {dung} \right)\\
C.\overrightarrow {BC} = 2\overrightarrow {MN} \Rightarrow C\,sai\\
20/\\
\overrightarrow {BA} + \overrightarrow {BC} = \overrightarrow {BG} + \overrightarrow {GA} + \overrightarrow {BG} + \overrightarrow {GC} = 2\overrightarrow {BG} + \left( {\overrightarrow {GA} + \overrightarrow {GC} } \right) = 2\overrightarrow {BG} + \left( { - \overrightarrow {GB} } \right) = 2\overrightarrow {BG} + \overrightarrow {BG} = 3\overrightarrow {BG}
\end{array}$
