High School Math : Law of Sines

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #1 : Triangles

 

 

Rt_triangle_letters
In this figure, angle \displaystyle a=30^\circ and side \displaystyle Z=15. If angle \displaystyle b=45^\circ, what is the length of side \displaystyle Y?

Possible Answers:

\displaystyle \frac{15\sqrt{2}}{2}

\displaystyle 7.5

\displaystyle 30\sqrt{2}

\displaystyle 15\sqrt{2}

\displaystyle 30

Correct answer:

\displaystyle 15\sqrt{2}

Explanation:

For this problem, use the law of sines:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}=\frac{\text{side opposite }c}{\sin(c)}.

In this case, we have values that we can plug in:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}

\displaystyle \frac{Z}{\sin(a)}=\frac{Y}{\sin{(b)}}

\displaystyle \frac{15}{\sin(30^\circ)}=\frac{Y}{\sin{(45^\circ)}}

\displaystyle \frac{15}{\frac{1}{2}}=\frac{Y}{\frac{\sqrt{2}}{2}}

Cross multiply:

\displaystyle 15*\frac{\sqrt{2}}{2}=\frac{1}{2}Y

Multiply both sides by \displaystyle 2:

\displaystyle 15\sqrt{2}=Y

Example Question #2 : Triangles

Rt_triangle_letters

In this figure \displaystyle a=22^\circ and \displaystyle c=85^\circ. If \displaystyle X=30, what is \displaystyle Z?

Possible Answers:

\displaystyle 0.997

\displaystyle 11.28

\displaystyle 0.374

\displaystyle 20.1

\displaystyle 30.09

Correct answer:

\displaystyle 11.28

Explanation:

For this problem, use the law of sines:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}=\frac{\text{side opposite }c}{\sin(c)}.

In this case, we have values that we can plug in:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }c}{\sin(c)}

\displaystyle \frac{Z}{\sin(a)}=\frac{X}{\sin{(c)}}

\displaystyle \frac{Z}{\sin(22^\circ)}=\frac{30}{\sin{85^\circ}}

\displaystyle \frac{Z}{0.375}=\frac{30}{0.997}

\displaystyle \frac{Z}{0.375}=30.09

\displaystyle Z=30.09*0.375

\displaystyle Z=11.28

Example Question #3 : Applying The Law Of Sines

In \displaystyle \Delta ABC\displaystyle m \angle A = 75^{\circ } , \displaystyle m \angle B = 62^{\circ }, and \displaystyle BC = 16. To the nearest tenth, what is \displaystyle AB?

Possible Answers:

\displaystyle 22.7

\displaystyle 27.9

\displaystyle 11.3

\displaystyle 14.6

\displaystyle 17.5

Correct answer:

\displaystyle 11.3

Explanation:

Since we are given \displaystyle BC and want to find \displaystyle AB, we apply the Law of Sines, which states, in part,

\displaystyle \frac{ \sin m \angle A}{B C} = \frac{ \sin m \angle C}{AB}

\displaystyle m \angle A = 75^{\circ } and

\displaystyle m \angle C = 180 - \left ( m \angle A + m \angle B \right )= 180 -(75+ 62) = 180 -137 = 43^{\circ }

Substitute in the above equation:

\displaystyle \frac{ \sin75^{\circ }}{16} = \frac{ \sin 43^{\circ }}{AB}

Cross-multiply and solve for \displaystyle AB:

\displaystyle AB\cdot \sin75^{\circ } = 16 \cdot \sin 43^{\circ }

Example Question #4 : Applying The Law Of Sines

In \displaystyle \Delta ABC\displaystyle m \angle A = 66^{\circ } , \displaystyle AB = 16, and \displaystyle BC = 23. To the nearest tenth, what is \displaystyle m \angle C?

Possible Answers:

\displaystyle 39.5^{\circ } \textrm{ or } 140.5^{\circ }

\displaystyle 73.6^{\circ }

No triangle can exist with these characteristics.

\displaystyle 39.5^{\circ }

\displaystyle 73.6^{\circ } \textrm{ or }106.4^{\circ }

Correct answer:

\displaystyle 39.5^{\circ }

Explanation:

Since we are given \displaystyle m \angle A , \displaystyle AB, and \displaystyle BC, and want to find \displaystyle m \angle C, we apply the Law of Sines, which states, in part,

\displaystyle \frac{ \sin m \angle C}{AB} = \frac{ \sin m \angle A}{B C}.

Substitute and solve for \displaystyle \sin m \angle C:

\displaystyle \frac{ \sin m \angle C}{16} = \frac{ \sin 66^{\circ }}{23}

\displaystyle \frac{ \sin m \angle C}{16} \cdot 16= \frac{ \sin 66^{\circ }}{23}\cdot 16

\displaystyle \sin m \angle C = \frac{ \sin 66^{\circ }}{23}\cdot 16 \approx \frac{ 0.9135}{23}\cdot 16\approx 0.6355

Take the inverse sine of 0.6355:

\displaystyle m \angle C = \sin^{-1} 0.6355 \approx 39.5^{\circ }

There are two angles between \displaystyle 0^{\circ } and \displaystyle 180^{\circ } that have any given positive sine other than 1 - we get the other by subtracting the previous result from \displaystyle 180^{\circ }:

\displaystyle 180 - 39.5 = 140.5^{\circ }

This, however, is impossible, since this would result in the sum of the triangle measures being greater than \displaystyle 180^{\circ }. This leaves \displaystyle 39.5^{\circ } as the only possible answer.

Example Question #1 : Law Of Sines

Rt_triangle_letters

In this figure, angle \displaystyle a=30^\circ. If side \displaystyle Z =12 and \displaystyle Y=20, what is the value of angle \displaystyle b?

Possible Answers:

Undefined

\displaystyle 90^\circ

\displaystyle 24^\circ

\displaystyle 0.83^\circ

\displaystyle 56.44^\circ

Correct answer:

\displaystyle 56.44^\circ

Explanation:

For this problem, use the law of sines:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}=\frac{\text{side opposite }c}{\sin(c)}.

In this case, we have values that we can plug in:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}

\displaystyle \frac{Z}{\sin(a)}=\frac{Y}{\sin{(b)}}

\displaystyle \frac{12}{\sin(30^\circ)}=\frac{20}{\sin{(b)}}

\displaystyle 24=\frac{20}{\sin(b)}

\displaystyle \sin(b)=\frac{20}{24}

\displaystyle b=\sin^{-1}(\frac{20}{24})

\displaystyle b=56.44^\circ

Example Question #2 : Graphing The Sine And Cosine Functions

Rt_triangle_lettersIn this figure, if angle \displaystyle a=18.5^\circ, side \displaystyle Z =30.2, and side \displaystyle Y=17.2, what is the value of angle \displaystyle b?

(NOTE: Figure not necessarily drawn to scale.)

Possible Answers:

\displaystyle 71.5^\circ

\displaystyle 61.22^\circ

\displaystyle 33.6^\circ

\displaystyle 10.41^\circ

Undefined

Correct answer:

\displaystyle 10.41^\circ

Explanation:

First, observe that this figure is clearly not drawn to scale. Now, we can solve using the law of sines:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}=\frac{\text{side opposite }c}{\sin(c)}.

In this case, we have values that we can plug in:

\displaystyle \frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}

\displaystyle \frac{Z}{\sin(a)}=\frac{Y}{\sin{(b)}}

\displaystyle \frac{30.2}{\sin(18.5^\circ)}=\frac{17.2}{\sin{(b)}}

\displaystyle 95.18=\frac{17.2}{\sin(b)}

\displaystyle \sin(b)=\frac{17.2}{95.18}

\displaystyle b=\sin^{-1}(\frac{17.2}{95.18})

\displaystyle b=10.41^\circ

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