Sketch the isoclines for the equations:

\[
\begin{aligned}
& \text { (a) } \frac{d y}{d x}=x y(y-2) \\
& \text { (b) } \ddot{x}+\omega_{n}^{2} x+\mu x^{3}=0 \\
& \text { (c) } \ddot{x}-\dot{x}\left(1-x^{2}ight)+x=0 .
\end{aligned}
\]