Precalculus (6th Edition)

Published by Pearson
ISBN 10: 013421742X
ISBN 13: 978-0-13421-742-0

Chapter 7 - Trigonometric Identities and Equations - 7.4 Double-Angle and Half-Angle Identities - 7.4 Exercises: 74

Answer

$=\cos {10}\alpha $

Work Step by Step

$\cos \dfrac {A}{2}=\pm \sqrt {\dfrac {1+\cos A}{2}}\Rightarrow \pm \sqrt {\dfrac {1+\cos {20}\alpha }{2}}=\cos \dfrac {20\alpha }{2}=\cos {10}\alpha $
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