Answer
$$\cos 2A =2\cos^2A-1$$
Work Step by Step
We find:
\begin{align*}
\cos 2A&=\cos^2A-\sin^2A,\ \ \ \ \ \ \ \ \ \text{Use }\cos^2x+\sin^2x=1 \\
&=\cos^2A-(1-\cos^2A)\\
&=2\cos^2A-1
\end{align*}
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 6 - Section 6.1 - Solving Trigonometric Equations - 6.1 Problem Set - Page 327: 78
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