Precalculus (6th Edition)

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

Chapter 8 - Application of Trigonometry - 8.2 The Law of Cosines - 8.2 Exercises - Page 766: 11



Work Step by Step

From the law of cosines $7^{2}=3^{2}+5^{2}-2\times 3\times 5\times \cos \theta \Rightarrow 49=34-30\times \cos \theta \Rightarrow -30\cos \theta =15\Rightarrow \cos \theta =-\dfrac {1}{2}\Rightarrow \theta =\cos ^{-1}\left( -\dfrac {1}{2}\right) =120^0$
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