$(a)\;0\qquad(b)\;cos (\large\frac{\pi}{6})+i\;sin(\large\frac{\pi}{6})\qquad(c)\;cos (\large\frac{\pi}{4})+i\;sin(\large\frac{\pi}{4})\qquad(d)\;cos (\large\frac{\pi}{2})+i\;sin(\large\frac{\pi}{2})$
