Algebra 2 (1st Edition)

Published by McDougal Littell
ISBN 10: 0618595414
ISBN 13: 978-0-61859-541-9

Chapter 13, Trigonometric Ratios and Functions - 13.3 Evaluate Trigonometric Functions of Any Angle - 13.3 Exercises - Skill Practice - Page 871: 28


$\dfrac{\sqrt 2}{2}$

Work Step by Step

When $P$ is the period of $f(x)$ then we have $ f(x+p)=f(x)$ Since the function cosine is positive in the fourth quadrant and $\dfrac{7 \pi}{4}$ is also positive in the same quadrant. Thus, the reference angle is: $\dfrac{\pi}{4}$. Here, we have $ \cos (\dfrac{7 \pi}{4})=\cos (\dfrac{\pi}{4})$ and $\cos (\dfrac{\pi}{4})=\dfrac{\sqrt 2}{2}$
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