Given
y=rsin(wt-$0)
let
U=wt-$
du/dt=w...................
y=rsinu.........................dy/du=rcosu............=rcos(wt-$)
from the chain rule
dy/dt=dy/du*du/dt................=rcos(wt-$)*w....................=rwcos(wt-$)
d(rsin(wt-$)/dt=rwcos(wt-$)