Answer
$\frac{\sqrt 5}{5}$
Work Step by Step
Recall: $\cos(\arctan x)=\frac{1}{\sqrt {1+x^{2}}}$
$\implies\cos(\arctan (2))=\frac{1}{\sqrt {1+2^{2}}}=\frac{1}{\sqrt {5}}=\frac{\sqrt 5}{5}$
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 5.4 - Half-Angle Formulas - 5.4 Problem Set - Page 305: 59
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