Precalculus (6th Edition)

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: 28

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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