Precalculus (10th Edition)

Published by Pearson
ISBN 10: 0-32197-907-9
ISBN 13: 978-0-32197-907-0

Chapter 7 - Analytic Trigonometry - Chapter Review - Review Exercises - Page 506: 49



Work Step by Step

Recall: $\cos(\alpha-\beta)=\cos(\alpha)\cos(\beta)+\sin(\alpha)\sin(\beta)$ Hence, $\cos(80^\circ)\cos(20^\circ)+\sin(80^\circ)\sin(20^\circ)\\ =\cos(80^\circ-20^\circ)\\ =\cos(60^\circ)\\ =\frac{1}{2}$
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