Answer
$-\frac{3}{5}$
Work Step by Step
Given $sin\alpha=-\frac{4}{5}$ and $\alpha$ in quadrant III, we have $y=-4, r=5$ and $x=-\sqrt {5^2-(-4)^2}=-3$, thus $cos\alpha=-\frac{3}{5}$
Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)
by Sullivan III, Michael
Published by
Pearson
ISBN 10:
0-32193-104-1
ISBN 13:
978-0-32193-104-7
Chapter 6 - Analytic Trigonometry - Section 6.5 Sum and Difference Formulas - 6.5 Assess Your Understanding - Page 508: 4
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