Answer
$\cos \left( -\dfrac {\pi }{8}\right) =\cos \dfrac {\pi }{8}$
Work Step by Step
$\sin \left( \dfrac {5\pi }{8}\right) =\cos \left( \dfrac {\pi }{2}-\dfrac {5\pi }{8}\right) =\cos \left( -\dfrac {\pi }{8}\right) =\cos \dfrac {\pi }{8}$
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