#### Answer

$\cos \left( -\dfrac {\pi }{6}\right) $

#### Work Step by Step

$\alpha +\beta =\dfrac {\pi }{2}\Rightarrow \sin \alpha =\cos \beta \Rightarrow \dfrac {2\pi }{3}+\left( -\dfrac {\pi }{6}\right) =\dfrac {\pi }{2}\Rightarrow $
$\sin \left( \dfrac {2\pi }{3}\right) =\cos \left( -\dfrac {\pi }{6}\right) $