Example 4. Determine the positional relationship of the following pairs of lines:
(1)
$$
\begin{array}{l}
l_{1}: y=\frac{4}{3} x+8, \\
l_{2}: x=\frac{3}{4} y+6
\end{array}
$$
$$
\text { (2) } \begin{aligned}
l_{3}: x & =\frac{3}{4} y+6, \\
l_{4}: y & =-\frac{3}{4} x+5 .
\end{aligned}
$$
$$
\begin{array}{l}
\text { Solution (1) } k_{x 1} \cdot k_{y 2}=\frac{4}{3} \cdot \frac{3}{4}=1, \\
\therefore l_{1} \| l_{2} \text {. }
\end{array}
$$
(2) $k_{y_{3}}=\frac{8}{4}, k_{x_{4}}=-\frac{3}{4}=-k_{y_{3}}$
$$
\therefore l_{3} \perp l_{4} .
$$