Answer
$a$
Work Step by Step
$\cos{\alpha} + \cos{\beta} = 2 \cos{\dfrac{\alpha+\beta}{2}} \cos{\dfrac{\alpha-\beta}{2}} $
$\cos{6x} + \cos{4x} = 2 \cos{5x} \cos{x}$
Trigonometry 7th Edition
by McKeague, Charles P.; Turner, Mark D.
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 312: 57
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