Answer
See below.
Work Step by Step
Letting $\theta=2u$, we have $LHS=cos^2\theta-sin^2\theta=cos(2\theta)=cos(4u)=RHS$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 7 - Analytic Trigonometry - 7.6 Double-angle and Half-angle Formulas - 7.6 Assess Your Understanding - Page 498: 55
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