Answer
$\frac{\sqrt 3}{2}$
Work Step by Step
Using the double-angle formula $cos2\theta=cos^2\theta-sin^2\theta$, we have $cos^215^\circ -sin^215^\circ=cos2(15^\circ)=cos30^\circ=\frac{\sqrt 3}{2}$
Precalculus (6th Edition) Blitzer
by Blitzer, Robert F.
Published by
Pearson
ISBN 10:
0-13446-914-3
ISBN 13:
978-0-13446-914-0
Chapter 5 - Review Exercises - Page 708: 39
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