Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 1 - Precalculus Review - 1.4 Trigonometric Functions - Exercises - Page 31: 51


$$\cos (\theta+\pi) =-\cos \theta$$

Work Step by Step

Since \begin{aligned} \cos (\theta+\pi) &=\cos \theta \cos \pi-\sin \theta \sin \pi \\ &=\cos \theta(-1)-\sin \theta(0) \\ \end{aligned} Then $$\cos (\theta+\pi) =-\cos \theta$$
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