Answer
$0$
Work Step by Step
Recall that $\cos(\alpha+\beta)=\cos(\alpha)\cos(\beta)-\sin(\alpha)\sin(\beta)$.
Hence,
$\cos(70^\circ)\cos(20^\circ)-\sin(70^\circ)\sin(20^\circ)\\
=\cos(70^\circ+20^\circ)\\
=\cos(90^\circ)\\
=0$
Precalculus (10th Edition)
by Sullivan, Michael
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
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