Answer
$-\sin\theta$
Work Step by Step
Use the subtraction formula for cosine, $\cos (A-B)=\cos A\cos B+\sin A\sin B$:
$\cos(\theta-270^\circ)$
$=\cos\theta\cos 270^\circ+\sin\theta\sin 270^\circ$
$=\cos\theta*0+\sin\theta*(-1)$
$=-\sin\theta$
Precalculus (6th Edition)
by Lial, Margaret L.; Hornsby, John; Schneider, David I.; Daniels, Callie
Published by
Pearson
ISBN 10:
013421742X
ISBN 13:
978-0-13421-742-0
Chapter 7 - Trigonometric Identities and Equations - 7.3 Sum and Difference Identities - 7.3 Exercises - Page 680: 64
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