Answer
$$\cos 2\theta =2\cos^2\theta-1$$
Work Step by Step
We use the know identities to obtain:
\begin{align*}
\cos 2\theta &=\cos^2\theta-\sin^2\theta\\
&= \cos^2\theta-(1-\cos^2\theta)\\
&=2\cos^2\theta-1
\end{align*}
Calculus (3rd Edition)
by Rogawski, Jon; Adams, Colin
Published by
W. H. Freeman
ISBN 10:
1464125260
ISBN 13:
978-1-46412-526-3
Chapter 1 - Precalculus Review - 1.4 Trigonometric Functions - Exercises - Page 31: 47
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