Answer
$cos~2θ~cos~4θ=\frac{1}{2}(cos~2θ+cos~6θ)$
Work Step by Step
$cos~2θ~cos~4θ=\frac{1}{2}[cos(2θ-4θ)+cos(2θ+4θ)]=\frac{1}{2}[cos(-2θ)+cos(6θ)]=\frac{1}{2}(cos~2θ+cos~6θ)$
Algebra and Trigonometry 10th Edition
by Larson, Ron
Published by
Cengage Learning
ISBN 10:
9781337271172
ISBN 13:
978-1-33727-117-2
Chapter 7 - 7.5 - Multiple-Angle and Product-to-Sum Formulas - 7.5 Exercises - Page 548: 51
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