Answer
$-\dfrac{1}{2}$
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)
Precalculus (10th Edition)
by Sullivan, Michael
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
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