#### Answer

$-1$

#### Work Step by Step

$\cos \dfrac {7\pi }{9}\cos \dfrac {2\pi }{9}-\sin \dfrac {7\pi }{9}\sin \dfrac {2\pi }{9}=\cos \left( \dfrac {7\pi }{9}+\dfrac {2\pi }{9}\right) =\cos \pi =-1$

Published by
Pearson

ISBN 10:
013421742X

ISBN 13:
978-0-13421-742-0

$-1$

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