Đáp án: $D\left( {25;27} \right)/D\left( { - 23; - 21} \right)$
Giải thích các bước giải:
$\begin{array}{l}
D\left( {x;y} \right)\\
Do:D \in d:x - y + 2 = 0\\
\Rightarrow y = x + 2\\
\Rightarrow D\left( {x;x + 2} \right)\\
\Rightarrow \left\{ \begin{array}{l}
\overrightarrow {AD} = \left( {x - 1;x + 2 - 3} \right) = \left( {x - 1;x - 1} \right)\\
\overrightarrow {AC} = \left( {4; - 3} \right)
\end{array} \right.\\
\Rightarrow \overrightarrow {AD} .\overrightarrow {AC} = 4.\left( {x - 1} \right) - 3.\left( {x - 1} \right) = x - 1\\
Do:{S_{ACD}} = \dfrac{1}{2}.\left| {\overrightarrow {AD} .\overrightarrow {AC} } \right| = 12\\
\Rightarrow \left| {x - 1} \right| = 24\\
\Rightarrow \left[ \begin{array}{l}
x - 1 = 24\\
x - 1 = - 24
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 25\\
x = - 23
\end{array} \right.\\
\Rightarrow D\left( {25;27} \right)/D\left( { - 23; - 21} \right)
\end{array}$
