Answer
$cos~120°+cos~60°=0$
Work Step by Step
$cos~120°+cos~60°=2~cos(\frac{120°+60°}{2})~cos(\frac{120°-60°}{2})=2~cos~90°~cos~30°=2(0)(\frac{\sqrt 3}{2})=0$
Algebra and Trigonometry 10th Edition
by Larson, Ron
Published by
Cengage Learning
ISBN 10:
9781337271172
ISBN 13:
978-1-33727-117-2
Chapter 7 - 7.5 - Multiple-Angle and Product-to-Sum Formulas - 7.5 Exercises - Page 549: 58
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