Answer
See below.
Work Step by Step
$LHS=cos^4\theta-sin^4\theta=(cos^2\theta+sin^2\theta)(cos^2\theta-sin^2\theta)=(1)(cos2\theta)=cos(2\theta)=RHS$
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: 47
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