Trigonometry 7th Edition

Published by Cengage Learning
ISBN 10: 1111826854
ISBN 13: 978-1-11182-685-7

Chapter 7 - Test - Page 415: 6

Answer

\[ 8.12 \mathrm{m}=c, 43.63^{\circ}=A, 17.37^{\circ}=B \]

Work Step by Step

\[ a=6.4 \mathrm{m}, b=2.8 \mathrm{m}, C=119^{\circ} \] We know the two sides and the angle between them. Thus we can use the Law of cosines to find the length of $c$. \[ a^{2}+b^{2}-2 a b \cos C=c^{2} \] Substitute $a, b$ and $C$ \[ \begin{aligned} &a^{2}+b^{2}-2 a b \cos C=c^{2} \\ &=6.4^{2}+2.8^{2}-2 \cdot 6.4 \cdot 2.8 \cos 119^{\circ} \\ &=40.96+7.84-35.84 \cdot(-0.48) \\ 66=c^{2} & \\ &=48.8+17.2 \\ & \\ \begin{array}{l} 66=c^{2} \\ \sqrt{66}=c \\ 8.12 \mathrm{m}=c \end{array} \end{aligned} \] Angle A is: \[ \begin{array}{l} \frac{\sin C}{c}=\frac{\sin A}{a} \\ \frac{\sin 119^{\circ}}{8.12} =\frac{\sin A}{6.4}\\ 6.69 =\sin A\\ \frac{0.87}{8.12}=\frac{\sin A}{6.4} \\ \sin ^{-1} 0.69=\qquad A \\ 0.69=\sin A \\ 43.63^{\circ}=\qquad A \end{array} \] The sum of all angles in a triangle is $180^{\circ}$ \[ 180^{\circ}=A+B+C \] \[ \begin{array}{c} 180^{\circ}=119^{\circ} +43.63^{\circ}+B\\ B=180^{\circ}-43.63^{\circ}-119^{\circ} \\ \hline B=17.37^{\circ} \end{array} \]
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