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: 99



Work Step by Step

1. With $-90^{\circ}\lt \theta\lt 90^{\circ}$, then $-270^{\circ}\lt \theta -180^{\circ}\lt -180^{\circ}$ 2. This means that $\theta -180^{\circ}$ is in QII or QIII, where $x\lt0$ 3. Therefore $cos( \theta -180)^{\circ}\lt 0$
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