Answer
False
Work Step by Step
$\cos (30 ^{\circ} +20^{\circ}) =^{?} cos 30^{\circ} + \cos 20^{\circ}$
$\cos (50 ^{\circ}) =^{?} 0.8660254037+ 0.9396926208$
$ 0.6427876097 \ne 1.805718025$
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 2 - Acute Angles and Right Triangles - Section 2.3 Finding Trigonometric Function Values Using a Calculator - 2.3 Exercises - Page 65: 58
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