Prove (cosA)^4 - (sinA)^4 = cos2A

Algebra & Trigonometry with Analytic Geometry
13th Edition
ISBN:9781133382119
Author:Swokowski
Publisher:Swokowski
Chapter7: Analytic Trigonometry
Section7.5: Product-to-sum And Sum-to-product Formulas
Problem 14E
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Prove (cosA)^4 - (sinA)^4 = cos2A.

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