Precalculus (6th Edition) Blitzer

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

Chapter 6 - Section 6.2 - The Law of Cosines - Exercise Set - Page 730: 18

Answer

$A\approx21^\circ, B\approx32^\circ, C\approx127^\circ$

Work Step by Step

Step 1. Based on the given conditions, using the Law of Cosines, we have $c^2=a^2+b^2-2ab\ cosC$, or $(9)^2=(4)^2+(6)^2-2(4)(6)cos(C)$, thus $cosC\approx-0.6042$ and $C=cos^{-1}(-0.6042)\approx127^\circ$ Step 2. Using the Law of Sines, we have $\frac{SinA}{a}=\frac{SinC}{c}$, $SinA=\frac{4}{9}sin(127^\circ)\approx 0.3549$, thus $A=sin^{-1}(0.3549)\approx21^\circ$ Step 3. We can find the angle as $B=180^\circ-127^\circ-21^\circ=32^\circ$ Step 4. We solved the triangle with $A\approx21^\circ, B\approx32^\circ, C\approx127^\circ$
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