\text { For } \mu \in[0,1) \text { and } r \leq \frac{5 \pi}{2} \text { consider the differential equation } \dot{r}=-\sin (r)+\frac{2 r}{5 \pi} \mu \cos (\theta) \dot{\theta}=\cos (T)

(a) Suppose u = 0. Draw the phase portrait. (b) Show that also for u E (0, 1) the differential equation has a closed orbit. (c) Show that for u E (0, 1) the differential equation has in fact (at least) two closed orbits (in addition to the fixed point at the origin).