Answer
$\cos(-x)=\cos x $
Work Step by Step
1. Set up calculator in degrees
$\cos20^{\circ}\approx0.9397$
$\cos(-20)^{\circ}\approx0.9397$
2. Pick up more examples
$\cos60^{\circ}=0.5$
$cos(-60)^{\circ}=0.5$
Trigonometry (10th Edition)
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
0321671775
ISBN 13:
978-0-32167-177-6
Chapter 1 - Trigonometric Functions - Section 1.3 Trigonometric Functions - 1.3 Exercises - Page 28: 106
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