判断下列向量中哪些是共线的．

$$
\begin{gathered}
\alpha _1=(1,2,3), \alpha _2=(1,-2,3), \alpha _3=(1,0,2), \alpha _4=(-3,6,-9), \\
\alpha _5=(2,0,4), \alpha _6=(-1,-2,-3), \alpha _7=\left(\frac{1}{4}, \frac{2}{4}, \frac{3}{4}\right), \alpha _8=\left(\frac{1}{2},-1,-\frac{3}{2}\right) .
\end{gathered}
$$