Answer
$-\dfrac {\sqrt {3}}{2}$
Work Step by Step
$\cos \left( \dfrac {5\pi }{6}\right) =\cos \left( \pi -\dfrac {\pi }{6}\right) =\cos \pi \cos \dfrac {\pi }{6}+\sin \pi \sin \dfrac {\pi }{6}=\left( -1\right) \times \cos \dfrac {\pi }{6}+0\times \sin \dfrac {\pi }{6}=-\cos \dfrac {\pi }{6}=-\dfrac {\sqrt {3}}{2}$
