Answer
The graphs appear to coincide.
Work Step by Step
$$\cos 3x + \cos x = 2 \cos \frac{3x+x}{2}\cos \frac{3x-x}{2}=2\cos 2x \cos x$$
Precalculus (6th Edition) Blitzer
by Blitzer, Robert F.
Published by
Pearson
ISBN 10:
0-13446-914-3
ISBN 13:
978-0-13446-914-0
Chapter 5 - Section 5.4 - Product-to-Sum and Sum-to-Product Formulas - Exercise Set - Page 690: 48
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