Trigonometry 7th Edition

Published by Cengage Learning
ISBN 10: 1111826854
ISBN 13: 978-1-11182-685-7

Chapter 5 - Section 5.5 - Additional Identities - 5.5 Problem Set - Page 311: 31

Answer

$0$

Work Step by Step

$\cos{A} \cos{B} =\dfrac{1}{2}[\cos{(A+B)} + \cos{(A-B)}]$ $\cos{90} \cos{180} = \dfrac{1}{2}[\cos{270} +\cos{(-90)}] = 0$
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