Answer
$\frac{\sqrt{2+\sqrt2}}{2}$
Work Step by Step
Using the given identities and the known values: $\sin (5\pi/8)=\pm\sqrt{\frac{1-\cos(5\pi/4)}{2}}=\sqrt{\frac{1-(\frac{1}{\sqrt2})}{2}}=\frac{\sqrt{2+\sqrt2}}{2}$
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