Answer
See below.
Work Step by Step
Let $v=2u$,
$LHS=cos^2(2u)-sin^2(2u)=cos^2(v)-sin^2(v)=cos(2v)=cos(4u)=RHS$
Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)
by Sullivan III, Michael
Published by
Pearson
ISBN 10:
0-32193-104-1
ISBN 13:
978-0-32193-104-7
Chapter 6 - Analytic Trigonometry - Section 6.6 Double-angle and Half-angle Formulas - 6.6 Assess Your Understanding - Page 519: 53
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