Đáp án:
$\begin{array}{l}
a)\frac{x}{{ - 2}} = \frac{y}{3} = \frac{z}{5} = 1\\
\Rightarrow \left\{ \begin{array}{l}
x = 1.\left( { - 2} \right) = - 2\\
y = 1.3 = 3\\
z = 1.5 = 5
\end{array} \right.\\
b)\frac{{ - x}}{{12}} = \frac{y}{6} = \frac{z}{3} = 2\\
\Rightarrow \left\{ \begin{array}{l}
- x = 2.12 = 24 \Rightarrow x = - 24\\
y = 2.6 = 12\\
z = 2.3 = 6
\end{array} \right.\\
c)\frac{x}{2} = \frac{3}{{ - y}} = \frac{z}{4} = \frac{t}{{18}} = \frac{1}{2}\\
\Rightarrow \left\{ \begin{array}{l}
x = \frac{1}{2}.2 = 1\\
- y = 3:\frac{1}{2} = 6 \Rightarrow y = - 6\\
z = \frac{1}{2}.4 = 2\\
t = \frac{1}{2}.18 = 9
\end{array} \right.
\end{array}$
