Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 6 - Section 6.1 - The Law of Sines - Exercise Set - Page 723: 81


The obtuse angle $B\approx 127{}^\circ $.

Work Step by Step

Consider the given expression $\cos B=\frac{{{6}^{2}}+{{4}^{2}}-{{9}^{2}}}{2\cdot 6\cdot 4}$. $\begin{align} & \cos B=\frac{{{6}^{2}}+{{4}^{2}}-{{9}^{2}}}{2\cdot 6\cdot 4} \\ & \cos B=\frac{36+16-81}{48} \\ & \cos B=-\frac{29}{48} \\ & B={{\cos }^{-1}}\left( \frac{-29}{48} \right) \end{align}$ This implies $B\approx 127{}^\circ $.
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