Answer
$-\frac{3}{5}$
Work Step by Step
As $\alpha$ is in quadrant III, $cos\alpha$ is negative. With $sin\alpha=-\frac{4}{5}$, we have $cos\alpha=-\sqrt {1-sin^2\alpha}=-\frac{3}{5}$
Precalculus (10th Edition)
by Sullivan, Michael
Published by
Pearson
ISBN 10:
0-32197-907-9
ISBN 13:
978-0-32197-907-0
Chapter 7 - Analytic Trigonometry - 7.5 Sum and Difference Formulas - 7.5 Assess Your Understanding - Page 487: 4
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