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: 27

Answer

$0$

Work Step by Step

Recall that $\cos(\alpha+\beta)=\cos(\alpha)\cos(\beta)-\sin(\alpha)\sin(\beta)$. Hence, $\cos(70^\circ)\cos(20^\circ)-\sin(70^\circ)\sin(20^\circ)\\ =\cos(70^\circ+20^\circ)\\ =\cos(90^\circ)\\ =0$
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