Đáp án:
$\begin{array}{l}
3)M\left( {x;y} \right)\\
\overrightarrow {AM} = - 2\overrightarrow {AB} \\
\Rightarrow \left( {x + 3;y - 2} \right) = - 2.\left( {1 + 3;4 - 2} \right)\\
\Rightarrow \left\{ \begin{array}{l}
x + 3 = - 2.4\\
y - 2 = - 2.2
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
x = - 11\\
y = - 2
\end{array} \right.\\
\Rightarrow M\left( { - 11; - 2} \right)\\
4)A\,sai\,\,do:\overrightarrow {DB} = 2\overrightarrow {DO} = - 2\overrightarrow {OD}
\end{array}$
