Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 5 - Section 5.3 - Double-Angle, Power-Reducing, and Half-Angle Formulas - Exercise Set - Page 680: 17

Answer

$-\frac{\sqrt 3}{2}$

Work Step by Step

Using the Double Angle Formula in reverse, $cos^2\theta - sin^2\theta =cos2\theta$ we have $cos^275^\circ - sin^275^\circ =cos2(75^\circ)=cos150^\circ=-\frac{\sqrt 3}{2}$
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