Precalculus (10th Edition)

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

Chapter 6 - Trigonometric Functions - Chapter Review - Chapter Test - Page 436: 9



Work Step by Step

We have: $\cos(-120)=-\cos 60=-\dfrac{1}{2}$ (we used the reference angle $60$ degrees and the minus sign because $-120$ degrees is in Quadrant III)
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