Precalculus (10th Edition)

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

Chapter 7 - Analytic Trigonometry - 7.5 Sum and Difference Formulas - 7.5 Assess Your Understanding - Page 488: 52



Work Step by Step

Use the identity $\cos{(\alpha-\beta)}=\cos{(\alpha)}\cos{(\beta)}+\sin{(\alpha)}\sin{(\beta)}$ to obtain: $\cos{(\pi+\theta)}\\=\cos{(\pi)}\cos{(\theta)}+\sin{(\pi)}\sin{(\theta)}\\=-1\cos{(\theta)}+0\cdot \sin{(\theta)}\\=-\cos{(\theta)}.$
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