Answer
$-\frac{1}{2}$
Work Step by Step
$\cos(-x)=\cos x$
$\implies \cos(-120^{\circ})=\cos(120^{\circ})$
As $\cos(180-x)^{\circ}=-\cos x$,
$\cos 120^{\circ}=\cos(180-60)^{\circ}=-\cos 60^{\circ}=-\frac{1}{2}$
Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)
by Sullivan III, Michael
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
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