Trigonometry (10th Edition)

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

Chapter 5 - Trigonometric Identities - Section 5.3 Sum and Difference Identities for Cosine - 5.3 Exercises - Page 212: 18

Answer

Use a scientific calculator to evaluate the given expression and obtain: $\cos{\left(\frac{7\pi}{9}\right)}\cos{\left(\frac{2\pi}{9}\right)}-\sin{\left(\frac{7\pi}{9}\right)}\sin{\left(\frac{2\pi}{9}\right)}=-1$

Work Step by Step

Use a scientific calculator to evaluate the given expression to obtain: $\cos{\left(\frac{7\pi}{9}\right)}\cos{\left(\frac{2\pi}{9}\right)}-\sin{\left(\frac{7\pi}{9}\right)}\sin{\left(\frac{2\pi}{9}\right)}=-1$
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