Đáp án:
$\begin{array}{l}
1)D\left( {x;y} \right) \Rightarrow \overrightarrow {DC} = \left( {2 - x; - y} \right)\\
\overrightarrow {AB} = \overrightarrow {DC} \\
\Rightarrow \left( {4;6} \right) = \left( {2 - x; - y} \right)\\
\Rightarrow \left\{ \begin{array}{l}
4 = 2 - x\\
6 = - y
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
x = - 2\\
y = - 6
\end{array} \right.\\
\Rightarrow D\left( { - 2; - 6} \right)\\
2)\\
a)\overrightarrow u + 3\overrightarrow v \\
\Rightarrow \left\{ \begin{array}{l}
x = 5 + 3.3 = 14\\
y = - 2 + 3.1 = 1
\end{array} \right.\\
\Rightarrow \overrightarrow u + 3\overrightarrow v = \left( {14;1} \right)\\
b)2\overrightarrow u + 5\overrightarrow v = \left( {25;1} \right)\\
3)\sqrt {2x + 1} = 3\left( {dk:x \ge - \frac{1}{2}} \right)\\
\Rightarrow 2x + 1 = 9\\
\Rightarrow x = 4\left( {tm} \right)
\end{array}$
