Precalculus (10th Edition)

Published by Pearson
ISBN 10: 0-32197-907-9
ISBN 13: 978-0-32197-907-0

Chapter 7 - Analytic Trigonometry - 7.5 Sum and Difference Formulas - 7.5 Assess Your Understanding - Page 487: 28



Work Step by Step

Recall that $\cos(\alpha-\beta)=\cos(\alpha)\cos(\beta)+\sin(\alpha)\sin(\beta)$. Hence, $\cos(40^\circ)\cos(10^\circ)+\sin(40^\circ)\sin(10^\circ)\\ =\cos(40^\circ-10^\circ)\\ =\cos(30^\circ)\\ =\dfrac{\sqrt3}{2}$
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