Answer
False
Work Step by Step
1. Lets start by substituting values into expression:
$\cos90^{\circ}+\sin90^{\circ}=0+1=1$ (correct)
$\cos270^{\circ}+\sin270^{\circ}=0+(-1)=-1$(false)
$\cos60^{\circ}+\sin60^{\circ}\ne1$ another counter example
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 1.4 Using the Definitions of the Trigonometric Functions - 1.4 Exercises - Page 37: 83
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