Trigonometry (10th Edition)

Published by Pearson
ISBN 10: 0321671775
ISBN 13: 978-0-32167-177-6

Chapter 7 - Applications of Trigonometry and Vectors - Section 7.3 The Law of Cosines - 7.3 Exercises - Page 313: 14

Answer

The angles are as follows: $A = 61^{\circ}, B = 40.7^{\circ},$ and $C = 78.3^{\circ}$ The lengths of the sides are as follows: $a = 5.4, b = 4,$ and $c = 6$

Work Step by Step

Let $b=4$, and let $c = 6$. We can use the law of cosines to find $a$: $a^2 = b^2+c^2-2bc~cos~A$ $a = \sqrt{b^2+c^2-2bc~cos~A}$ $a = \sqrt{4^2+6^2-(2)(4)(6)~cos~61^{\circ}}$ $a = \sqrt{28.729}$ $a = 5.4$ We can use the law of cosines to find $B$: $b^2 = a^2+c^2-2ac~cos~B$ $2ac~cos~B = a^2+c^2-b^2$ $cos~B = \frac{a^2+c^2-b^2}{2ac}$ $B = arccos(\frac{a^2+c^2-b^2}{2ac})$ $B = arccos(\frac{(5.4)^2+6^2-4^2}{(2)(5.4)(6)})$ $B = arccos(0.7586)$ $B = 40.7^{\circ}$ We can find angle $C$: $A+B+C = 180^{\circ}$ $C = 180^{\circ}-A-B$ $C = 180^{\circ}-61^{\circ}-40.7^{\circ}$ $C = 78.3^{\circ}$
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