#### Answer

$-\tan \dfrac {2\pi }{5}$

#### Work Step by Step

$\cot \left( \dfrac {9\pi }{10}\right) =\tan \left( \dfrac {\pi }{2}-\dfrac {9\pi }{10}\right) =\tan \left( -\dfrac {2\pi }{5}\right) =-\tan \dfrac {2\pi }{5}$

Published by
Pearson

ISBN 10:
013421742X

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

$-\tan \dfrac {2\pi }{5}$

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