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