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 - Page 694: 74


$=\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 $
Update this answer!

You can help us out by revising, improving and updating this answer.

Update this answer

After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.