Use reference angles or the symmetry shown in Figure 59 to obtain:

(a) $\sin \dfrac{7\pi }{4}=-\dfrac{\displaystyle\sqrt{2}}{2}$

(b) $\cos \dfrac{7\pi }{6}=-\dfrac{\displaystyle\sqrt{3}}{2}$

(c) $\tan \dfrac{2\pi }{3}=\dfrac{\dfrac{\displaystyle\sqrt{3}}{2}}{-\dfrac{1}{2}}=-\displaystyle\sqrt{3}$

(d) $\cos \left( -\dfrac{3\pi }{4}\right) =-\dfrac{\displaystyle\sqrt{2}}{2}$

(e) $\sin \left( -\dfrac{\pi }{6}\right) =-\dfrac{1}{2}$

(f) $\sin \dfrac{7\pi }{3}=\dfrac{\displaystyle\sqrt{3}}{2}$