Sketch the phase portrait for the following systems:
(a) $\begin{aligned} & \dot{u}_1=u_1-3 u_2, \\ & \dot{u}_2=-3 u_1+u_2 .\end{aligned}$
(b)
$$
\begin{aligned}
& \dot{u}_1=u_1-4 u_2, \\
& \dot{u}_2=u_1-u_2 .
\end{aligned}
$$
(c) $\dot{u}_1=u_1+u_2$,
(d)
$$
\begin{aligned}
& \dot{u}_1=u_1+u_2, \\
& \dot{u}_2=u_2 .
\end{aligned}
$$
(e) $\dot{u}_1=\frac{3}{2} u_1+\frac{5}{2} u_2$.
$$
\dot{u}_2=-\frac{5}{2} u_1+\frac{3}{2} u_2 \text {. }
$$