Precalculus (6th Edition)

Published by Pearson
ISBN 10: 013421742X
ISBN 13: 978-0-13421-742-0

Chapter 7 - Trigonometric Identities and Equations - 7.3 Sum and Difference Identities - 7.3 Exercises - Page 679: 33



Work Step by Step

$\cos \alpha =\dfrac {1}{sec\alpha }=\dfrac {1}{csc\left( 90^{0}-\alpha \right) }\Rightarrow \cos 70=\dfrac {1}{sec70}=\dfrac {1}{csc20}$
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