Answer
$\dfrac {\sqrt {2}}{2}$
Work Step by Step
$\sin \alpha =\sin \left( \alpha -2\pi k\right)
\\\Rightarrow \sin \dfrac {9\pi }{4}=\sin \left( \dfrac {9\pi }{4}-2\pi \right)
\\=\sin \dfrac {\pi }{4}
\\=\dfrac {\sqrt {2}}{2}$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 6 - Trigonometric Functions - 6.3 Properties of the Trigonometric Functions - 6.3 Assess Your Understanding - Page 394: 20
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