Trigonometry : Law of Sines

Study concepts, example questions & explanations for Trigonometry

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Example Questions

Example Question #1 : Law Of Sines

Figure1

Given sides \displaystyle a = 10\displaystyle b = 5 and angle \displaystyle \angle A = 100^{\circ} determine the corresponding value for \displaystyle \angle B

 

Possible Answers:

\displaystyle 29.5^{\circ}

Undefined

\displaystyle 5^{\circ}

\displaystyle 60^{\circ}

Correct answer:

\displaystyle 29.5^{\circ}

Explanation:

The Law of Sines is used here since we have Side - Angle - Side. We setup our equation as follows:

\displaystyle \frac{a}{\sin A} = \frac{b}{\sin B}

Next, we substitute the known values:

\displaystyle \frac{10}{\sin 100} = \frac{5}{\sin B}

Now we cross multiply:

\displaystyle 10\sin B = 5\sin 100

Divide by 10 on both sides:

\displaystyle \sin B = 0.5\sin100

Finally taking the inverse sine to obtain the desired angle:

\displaystyle B = sin^{-1}\left(0.5\sin100\right ) = 29.5^{\circ}

Example Question #2 : Law Of Sines

Let  \displaystyle \angle A = 17^{\circ}\displaystyle \angle C = 110^{\circ} and \displaystyle a = 5, determine the length of side \displaystyle b.

Figure2

Possible Answers:

\displaystyle 1.83

\displaystyle 5.88

\displaystyle 5.21

\displaystyle 16.07

\displaystyle 13.66

Correct answer:

\displaystyle 13.66

Explanation:

We have two angles and one side, however we do not have \displaystyle \angle B. We can determine the angle using the property of angles in a triangle summing to \displaystyle 180^{\circ}:

\displaystyle \angle B = 180^{\circ} - 17^{\circ} - 110^{\circ} = 53^{\circ}

Now we can simply utilize the Law of Sines:

\displaystyle \frac{5}{\sin 17} = \frac{b}{\sin 53}

Cross multiply and divide:

\displaystyle \frac{5\sin 53}{\sin 17} = b

Reducing to obtain the final solution:

\displaystyle b = 13.66

Example Question #3 : Law Of Sines

Triangle

In the above triangle, \displaystyle a = 35^{\circ} and \displaystyle c = 93^{\circ}. If \displaystyle X = 49, what is \displaystyle Y to the nearest tenth? (note: triangle not to scale)

Possible Answers:

\displaystyle 38.7

\displaystyle 28.1

\displaystyle 67.3

\displaystyle 62.1

\displaystyle 35.7

Correct answer:

\displaystyle 38.7

Explanation:

If we solve for \displaystyle b, we can use the Law of Sines to find \displaystyle Y.

Since the sum of angles in a triangle equals \displaystyle 180^{\circ},

\displaystyle a + b + c = 180^{\circ}

\displaystyle 35^{\circ} + b + 93^{\circ} = 180^{\circ}

\displaystyle b = 52^{\circ}

 

Now, using the Law of Sines:

\displaystyle \frac{{X}}{\sin\left ( c\right )} = \frac{{Y}}{\sin\left ( b\right )}

\displaystyle \frac{{49}}{\sin\left ( 93^{\circ}\right )} = \frac{{Y}}{\sin\left ( 52^{\circ}\right )}

\displaystyle \frac{49}{0.999} \approx \frac{Y}{0.788}

\displaystyle Y \approx \frac{49 \cdot 0.788 }{0.999}

\displaystyle Y \approx 38.7

 

Example Question #2 : Law Of Sines

Screen_shot_2015-03-07_at_5.09.32_pm

By what factor is \displaystyle b larger than \displaystyle a in the triangle pictured above.

Possible Answers:

\displaystyle 1.8

\displaystyle 2

\displaystyle 1.51

\displaystyle 0.66

It isn't

Correct answer:

\displaystyle 1.51

Explanation:

The Law of Sines states

\displaystyle \frac{a}{\sin(A)}=\frac{b}{\sin(B)}=\frac{c}{\sin(C)}

so for a and b, that sets up

\displaystyle \frac{a}{\sin(35)}=\frac{b}{\sin(60)}, b=a\frac{\sin(60)}{\sin(35)}=1.51a

Example Question #2 : Law Of Sines

Solve for \displaystyle \theta:
Sines 1

Possible Answers:

\displaystyle 68.18 ^o

\displaystyle 47.5 ^ o

\displaystyle 0.15^o

\displaystyle 45.49^o

\displaystyle 7.24 ^ o

Correct answer:

\displaystyle 68.18 ^o

Explanation:

To solve, use the law of sines, \displaystyle \frac{ \sin A }{a } = \frac{ \sin B }{b} where a is the side across from the angle A, and b is the side across from the angle B.

\displaystyle \frac{ \sin \theta }{ 19 } = \frac{ \sin ( 20^o )}{ 7 } cross-multiply

\displaystyle 7 \sin \theta = 19 \cdot \sin (20^o ) evaluate the right side

\displaystyle 7 \sin \theta = 6.4984 divide by 7 

\displaystyle \sin \theta = 0.9283 take the inverse sine 

\displaystyle \theta = \sin ^ {-1 } (0.9283 ) \approx 68.18 ^o

 

Example Question #1 : Law Of Sines

Evaluate using law of sines:

Sines 2

Possible Answers:

\displaystyle 2.96

\displaystyle 7.72

\displaystyle 6.34

\displaystyle 16.56

\displaystyle 3.26

Correct answer:

\displaystyle 7.72

Explanation:

To solve, use law of sines, \displaystyle \frac{ \sin A }{a} = \frac{ \sin B }{b} where side a is across from angle A, and side b is across from angle B.

In this case, we have a 90-degree angle across from x, but we don't currently know the angle across from the side length 7. We can figure out this angle by subtracting \displaystyle 25^o from \displaystyle 90^o:

\displaystyle 90^o - 25 ^o = 65^o

Now we can set up and solve using law of sines:

\displaystyle \frac{ \sin (65^o )}{ 7 } = \frac{ \sin (90 ^o )} { x } cross-multiply

\displaystyle \sin (90 ^o ) \cdot 7 = \sin (65^o ) \cdot x evaluate the sines

\displaystyle 1 \cdot 7 = (0.9063) x divide by 0.9063

\displaystyle 7.72 = x

Example Question #1 : Law Of Sines

What is the measure of \displaystyle \angle F in \displaystyle \triangle DEF below? Round to the nearest tenth of a degree.

Triangle def

Possible Answers:

\displaystyle 18.2^\circ

\displaystyle 47.8^\circ

\displaystyle 20.0^\circ

\displaystyle 16.6^\circ

\displaystyle 0.3^\circ

Correct answer:

\displaystyle 16.6^\circ

Explanation:

The law of sines tells us that \displaystyle \frac{sinA}{a}=\frac{sinB}{b}=\frac{sinC}{c}, where ab, and c are the sides opposite of angles AB, and C. In \displaystyle \triangle DEF, these ratios can be used to find \displaystyle \angle F:

\displaystyle \frac{sinD}{d}=\frac{sinF}{f}

\displaystyle \frac{sin114^\circ}{16}=\frac{sin F}{5}

\displaystyle sinF=\frac{5sin114^\circ}{16}

\displaystyle F=sin^{-1}\left ( \frac{5sin114^\circ}{16}\right )

\displaystyle F\approx16.6^\circ

Example Question #1 : Law Of Sines

Find the length of the line segment \displaystyle \overline{AB} in the triangle below.

Round to the nearest hundredth of a centimeter.

Triangle

Possible Answers:

\displaystyle 10.83\ cm

\displaystyle 14.66\ cm

\displaystyle 13.84\ cm

\displaystyle 15.60\ cm

\displaystyle 12.21\ cm

Correct answer:

\displaystyle 13.84\ cm

Explanation:

The law of sines states that 

\displaystyle \frac{sinA}{a}=\frac{sinB}{b}=\frac{sinC}{c}.

In this triangle, we are looking for the side length c, and we are given angle A, angle B, and side b. The sum of the interior angles of a triangle is \displaystyle 180^{\circ}; using subtraction we find that angle C\displaystyle 57^{\circ}.

We can now form a proportion that includes only one unknown, c:

\displaystyle \frac{sin52^{\circ}}{13}=\frac{sin57^{\circ}}{c}

Solving for c, we find that 

\displaystyle c=\frac{13sin57^{\circ}}{sin52^{\circ}}\approx13.84.

Example Question #1 : Law Of Sines

In the triangle below, \displaystyle m\angle A=78^\circ\displaystyle m\angle B=40^\circ, and \displaystyle c=14. What is the length of side \displaystyle b to the nearest tenth?

Triangle abc

Possible Answers:

\displaystyle 10.2

\displaystyle 12.6

\displaystyle 15.5

\displaystyle 9.2

\displaystyle 19.2

Correct answer:

\displaystyle 10.2

Explanation:

First, find \displaystyle m\angle C. The sum of the interior angles of a triangle is \displaystyle 180^\circ, so \displaystyle m\angle C = 180^\circ - 78^\circ - 40^\circ, or \displaystyle 62^\circ.

Using this information, you can set up a proportion to find side b:

\displaystyle \frac{sinC}{c}=\frac{sinB}{b}

\displaystyle \frac{sin62^\circ}{14}=\frac{sin40^\circ}{b}

\displaystyle b=\frac{14sin40^\circ}{sin62^\circ}

\displaystyle b\approx10.2

 

Example Question #1 : Law Of Sines

In the triangle below, \displaystyle m\angle A=75^\circ\displaystyle m\angle B=21^\circ, and \displaystyle c=19.3.

Triangle abc

What is the length of side a to the nearest tenth?

Possible Answers:

\displaystyle 7.0

\displaystyle 52.0

\displaystyle 19.9

\displaystyle 7.2

\displaystyle 18.7

Correct answer:

\displaystyle 18.7

Explanation:

To use the law of sines, first you must find the measure of \displaystyle \angle C. Since the sum of the interior angles of a triangle is \displaystyle 180^\circ\displaystyle m\angle C=84^\circ.

Law of sines:

\displaystyle \frac{\sin A}{a}=\frac{\sin C}{c}

\displaystyle \frac{\sin 75^\circ}{a}=\frac{\sin 84^\circ}{19.3}

\displaystyle a=\frac{19.3 \sin 75^\circ}{\sin 84^\circ}

\displaystyle a\approx 18.7

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