Answer
$-\frac{\sqrt{2-\sqrt2}}{2}$
Work Step by Step
Using the given identities and the known values: $\sin (-22.5^\circ)=\pm\sqrt{\frac{1-\cos(-45^\circ)}{2}}=-\sqrt{\frac{1-(\frac{1}{\sqrt2})}{2}}=-\frac{\sqrt{2-\sqrt2}}{2}$
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