Answer
$-\dfrac {\sqrt {2}}{2}$
Work Step by Step
$2\cos ^{2}\alpha -1=\cos _{2}\alpha \Rightarrow 2\cos ^{2}67\dfrac {1}{2}-1=\cos \left( 2\times \left( 67\dfrac {1}{2}\right) \right) =\cos 135=-\dfrac {\sqrt {2}}{2}$
Precalculus (6th Edition)
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
013421742X
ISBN 13:
978-0-13421-742-0
Chapter 7 - Trigonometric Identities and Equations - 7.4 Double-Angle and Half-Angle Identities - 7.4 Exercises - Page 693: 25
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