Answer
$-1$
Work Step by Step
$\cos \dfrac {7\pi }{9}\cos \dfrac {2\pi }{9}-\sin \dfrac {7\pi }{9}\sin \dfrac {2\pi }{9}=\cos \left( \dfrac {7\pi }{9}+\dfrac {2\pi }{9}\right) =\cos \pi =-1$
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: 20
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