Answer
0
Work Step by Step
1. Distribute
$\cos[180^{\circ}n+90^{\circ}]$
2. If n is an integer, then all possible angles are coterminal with either 90 or 270 degree angles. Therefore:
$\cos90^{\circ}=\cos270^{\circ}=0$
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 27: 93
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