Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

Published by Pearson
ISBN 10: 0-32193-104-1
ISBN 13: 978-0-32193-104-7

Chapter 5 - Trigonometric Functions - Chapter Test - Page 460: 9



Work Step by Step

$\cos(-x)=\cos x$ $\implies \cos(-120^{\circ})=\cos(120^{\circ})$ As $\cos(180-x)^{\circ}=-\cos x$, $\cos 120^{\circ}=\cos(180-60)^{\circ}=-\cos 60^{\circ}=-\frac{1}{2}$
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