Precalculus (10th Edition)

Published by Pearson
ISBN 10: 0-32197-907-9
ISBN 13: 978-0-32197-907-0

Chapter 8 - Applications of Trigonometric Functions - 8.2 The Law of Sines - 8.2 Assess Your Understanding - Page 530: 17

Answer

$C=120°$ $b\approx1.06$ $c\approx2.69$

Work Step by Step

Sketch the triangle using the given measurements. Refer to the attached image below. The sum of the three angles inside a triangle is $180^o$ so $A+B+C=180°$ Here, $A=40°$ $B=20°$ $a=2$ Therefore, $C=180°−40°−20°\\ C=120°$ Now, by applying the Law of Sines we can calculate the values of the remaining two sides: $$\dfrac{\sin(A)}{a}=\dfrac{\sin(B)}{b}=\dfrac{\sin(C)}{c}$$ We know one side of the equation, which is: $$\dfrac{\sin(A)}{a}=\dfrac{\sin(40°)}{2}$$ First, we calculate for $b$: $$\dfrac{\sin(B)}{b}=\dfrac{\sin(A)}{a}\\ \dfrac{\sin(20°)}{b}=\dfrac{\sin40^o}{2}$$ Cross-multiply, then isolate $b$ to obtain: $$b\cdot\sin40^o=2\cdot \sin20^o\\ b=\dfrac{2\cdot \sin20^o}{\sin40^o}\\ b\approx1.06 $$ Next, we calculate for $c$: $$\dfrac{\sin(C)}{c}=\dfrac{\sin(A)}{a}\\ \frac{\sin(120°)}{c}=\dfrac{\sin40°}{2}$$ Cross-multiply, then isolate $c$ to obtain: $$c\cdot \sin40°=2\cdot \sin120°\\ c=\dfrac{2\cdot \sin120°}{\sin40°}\\ c\approx 2.69$$
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