Chapter 7 - Trigonometric Identities and Equations - 7.3 Sum and Difference Identities - 7.3 Exercises - Page 679: 30

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

$\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) $