答案：(a) $P’=(5,3)$, $Q’=(-19,-7)$
- $P'(5,3)$，$Q'(-19,-7)$。
- $PQ$ 的斜率
$\begin{array}{rcl}
m_{PQ} & = & \dfrac{5-(-7)}{(-3) – 2} \\
& = & \dfrac{-12}{5}
\end{array}$$P’Q’$ 的斜率
$\begin{array}{rcl}
m_{P’Q’} & = & \dfrac{3-(-7)}{5-(-19)} \\
& = & \dfrac{5}{12}
\end{array}$由於 $m_{PQ} \times m_{P’Q’} = -1$，所以 $PQ$ 垂直於 $P’Q’$。
