Answer
$\dfrac{-4}{5}$
Work Step by Step
$\cos{2x} = 2 \cos^2 {x} - 1 = 2 \left(\dfrac{1}{\sqrt{10}}\right)^2 -1 = \dfrac{-4}{5}$
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 5.3 - Double-Angle Formulas - 5.3 Problem Set - Page 296: 11
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