Trigonometry (10th Edition)

Published by Pearson
ISBN 10: 0321671775
ISBN 13: 978-0-32167-177-6

Chapter 1 - Trigonometric Functions - Section 1.4 Using the Definitions of the Trigonometric Functions - 1.4 Exercises - Page 37: 91



Work Step by Step

1. With $90^{\circ}\lt \theta\lt 180^{\circ}$, then $-180^{\circ}\lt -\theta \lt -90^{\circ}$ 2. This means that $-\theta $ is in QIII, where $x\lt0$ and $y\lt0$ 3. Therefore $\\cos( -\theta)\lt 0$
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