Answer
$ \frac{1}{2}$
Work Step by Step
$cos80^\circ cos20^\circ+sin80^\circ sin20^\circ=cos(80-20)^\circ
=cos(60)^\circ=\frac{1}{2}$
Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)
by Sullivan III, Michael
Published by
Pearson
ISBN 10:
0-32193-104-1
ISBN 13:
978-0-32193-104-7
Chapter 6 - Analytic Trigonometry - Chapter Review - Review Exercises - Page 528: 49
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