Answer
(a) $ 0^\circ$ (or $360^\circ$)
(b) $60^\circ$ and $300^\circ$
(c) $ 90^\circ$ and $270^\circ$
(d) $180^\circ$
Work Step by Step
(a) Given $F=0, 0^\circ\leq\theta\leq 360^\circ$, we have $cos\theta=1$,
and $\theta= 0^\circ$ or $360^\circ$ (they are identifcal)
(b) For $F=0.25$, we have $1-cos\theta=0.5$ so $cos\theta=0.5$, and
we can find $\theta= 60^\circ$ and $300^\circ$
(c) $F=0.5$, $cos\theta=0$ which gives $\theta= 90^\circ$ and $270^\circ$
(d) $F=1$, $cos\theta=-1$ which gives $\theta= 180^\circ$
