Answer
$0$
Work Step by Step
$\cos 40\cos 50-\sin 40\sin 50=\cos \left( 40+50\right) =\cos 90=0$
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 679: 19
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