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