text: ## Precalculus (6th Edition)
  
  $\cos \left( -\dfrac {\pi }{8}\right) =\cos \dfrac {\pi }{8}$
  $\sin \left( \dfrac {5\pi }{8}\right) =\cos \left( \dfrac {\pi }{2}-\dfrac {5\pi }{8}\right) =\cos \left( -\dfrac {\pi }{8}\right) =\cos \dfrac {\pi }{8}$