- Use the Law of Sines to solve oblique triangles.
- Find the area of an oblique triangle using the sine function.
- Solve applied problems using the Law of Sines.
Suppose two radar stations located 20 miles apart each detect an aircraft between them. The angle of elevation measured by the first station is 35 degrees, whereas the angle of elevation measured by the second station is 15 degrees. How can we determine the altitude of the aircraft? We see in Figure 1 that the triangle formed by the aircraft and the two stations is not a right triangle, so we cannot use what we know about right triangles. In this section, we will find out how to solve problems involving non-right triangles.
Using the Law of Sines to Solve Oblique Triangles
In any triangle, we can draw an altitude, a perpendicular line from one vertex to the opposite side, forming two right triangles. It would be preferable, however, to have methods that we can apply directly to non-right triangles without first having to create right triangles.
Any triangle that is not a right triangle is an oblique triangle. Solving an oblique triangle means finding the measurements of all three angles and all three sides. To do so, we need to start with at least three of these values, including at least one of the sides. We will investigate three possible oblique triangle problem situations:
- ASA (angle-side-angle) We know the measurements of two angles and the included side. See Figure 2.
- AAS (angle-angle-side) We know the measurements of two angles and a side that is not between the known angles. See Figure 3.
- SSA (side-side-angle) We know the measurements of two sides and an angle that is not between the known sides. See Figure 4.
Knowing how to approach each of these situations enables us to solve oblique triangles without having to drop a perpendicular to form two right triangles. Instead, we can use the fact that the ratio of the measurement of one of the angles to the length of its opposite side will be equal to the other two ratios of angle measure to opposite side. Let’s see how this statement is derived by considering the triangle shown in Figure 5.
Using the right triangle relationships, we know thatandSolving both equations forgives two different expressions for
We then set the expressions equal to each other.
Similarly, we can compare the other ratios.
Collectively, these relationships are called the Law of Sines.
Note the standard way of labeling triangles: angle(alpha) is opposite sideangle(beta) is opposite sideand angle(gamma) is opposite sideSee Figure 6.
While calculating angles and sides, be sure to carry the exact values through to the final answer. Generally, final answers are rounded to the nearest tenth, unless otherwise specified.
Given a triangle with angles and opposite sides labeled as in Figure 6, the ratio of the measurement of an angle to the length of its opposite side will be equal to the other two ratios of angle measure to opposite side. All proportions will be equal. The Law of Sines is based on proportions and is presented symbolically two ways.
To solve an oblique triangle, use any pair of applicable ratios.
Solving for Two Unknown Sides and Angle of an AAS Triangle
Solve the triangle shown in Figure 7 to the nearest tenth.
The three angles must add up to 180 degrees. From this, we can determine that
To find an unknown side, we need to know the corresponding angle and a known ratio. We know that angle and its corresponding sideWe can use the following proportion from the Law of Sines to find the length of
Similarly, to solve forwe set up another proportion.
Therefore, the complete set of angles and sides is
Using The Law of Sines to Solve SSA Triangles
We can use the Law of Sines to solve any oblique triangle, but some solutions may not be straightforward. In some cases, more than one triangle may satisfy the given criteria, which we describe as an ambiguous case. Triangles classified as SSA, those in which we know the lengths of two sides and the measurement of the angle opposite one of the given sides, may result in one or two solutions, or even no solution.
Oblique triangles in the category SSA may have four different outcomes. Figure 9 illustrates the solutions with the known sidesandand known angle
Solving an Oblique SSA Triangle
Solve the triangle in Figure 10 for the missing side and find the missing angle measures to the nearest tenth.
Use the Law of Sines to find angleand angleand then sideSolving forwe have the proportion
However, in the diagram, angleappears to be an obtuse angle and may be greater than 90°. How did we get an acute angle, and how do we find the measurement ofLet’s investigate further. Dropping a perpendicular fromand viewing the triangle from a right angle perspective, we have Figure 11. It appears that there may be a second triangle that will fit the given criteria.
The angle supplementary tois approximately equal to 49.9°, which means that(Remember that the sine function is positive in both the first and second quadrants.) Solving for we have
We can then use these measurements to solve the other triangle. Sinceis supplementary to the sum ofand we have
Now we need to findand
To summarize, there are two triangles with an angle of 35°, an adjacent side of 8, and an opposite side of 6, as shown in Figure 12.
However, we were looking for the values for the triangle with an obtuse angleWe can see them in the first triangle (a) in Figure 12.
Givenandfind the missing side and angles. If there is more than one possible solution, show both.
Solving for the Unknown Sides and Angles of a SSA Triangle
In the triangle shown in Figure 13, solve for the unknown side and angles. Round your answers to the nearest tenth.
In choosing the pair of ratios from the Law of Sines to use, look at the information given. In this case, we know the angleand its corresponding sideand we know sideWe will use this proportion to solve for
To findapply the inverse sine function. The inverse sine will produce a single result, but keep in mind that there may be two values forIt is important to verify the result, as there may be two viable solutions, only one solution (the usual case), or no solutions.
In this case, if we subtractfrom 180°, we find that there may be a second possible solution. Thus,To check the solution, subtract both angles, 131.7° and 85°, from 180°. This gives
which is impossible, and so
To find the remaining missing values, we calculateNow, only sideis needed. Use the Law of Sines to solve forby one of the proportions.
The complete set of solutions for the given triangle is
Givenfind the missing side and angles. If there is more than one possible solution, show both. Round your answers to the nearest tenth.
Finding the Triangles That Meet the Given Criteria
Find all possible triangles if one side has length 4 opposite an angle of 50°, and a second side has length 10.
Using the given information, we can solve for the angle opposite the side of length 10. See Figure 14.
We can stop here without finding the value ofBecause the range of the sine function isit is impossible for the sine value to be 1.915. In fact, inputtingin a graphing calculator generates an ERROR DOMAIN. Therefore, no triangles can be drawn with the provided dimensions.
Determine the number of triangles possible given
Finding the Area of an Oblique Triangle Using the Sine Function
Now that we can solve a triangle for missing values, we can use some of those values and the sine function to find the area of an oblique triangle. Recall that the area formula for a triangle is given aswhereis base andis height. For oblique triangles, we must findbefore we can use the area formula. Observing the two triangles in Figure 15, one acute and one obtuse, we can drop a perpendicular to represent the height and then apply the trigonometric propertyto write an equation for area in oblique triangles. In the acute triangle, we haveorHowever, in the obtuse triangle, we drop the perpendicular outside the triangle and extend the baseto form a right triangle. The angle used in calculation isor
The formula for the area of an oblique triangle is given by
This is equivalent to one-half of the product of two sides and the sine of their included angle.
Finding the Area of an Oblique Triangle
Find the area of a triangle with sidesand angleRound the area to the nearest integer.
Using the formula, we have
Find the area of the triangle givenRound the area to the nearest tenth.
Solving Applied Problems Using the Law of Sines
The more we study trigonometric applications, the more we discover that the applications are countless. Some are flat, diagram-type situations, but many applications in calculus, engineering, and physics involve three dimensions and motion.
Finding an Altitude
Find the altitude of the aircraft in the problem introduced at the beginning of this section, shown in Figure 16. Round the altitude to the nearest tenth of a mile.
To find the elevation of the aircraft, we first find the distance from one station to the aircraft, such as the side and then use right triangle relationships to find the height of the aircraft,
Because the angles in the triangle add up to 180 degrees, the unknown angle must be 180°−15°−35°=130°. This angle is opposite the side of length 20, allowing us to set up a Law of Sines relationship.
The distance from one station to the aircraft is about 14.98 miles.
Now that we knowwe can use right triangle relationships to solve for
The aircraft is at an altitude of approximately 3.9 miles.
The diagram shown in Figure 17 represents the height of a blimp flying over a football stadium. Find the height of the blimp if the angle of elevation at the southern end zone, point A, is 70°, the angle of elevation from the northern end zone, pointis 62°, and the distance between the viewing points of the two end zones is 145 yards.
10.1 Section Exercises
Describe the altitude of a triangle.
Compare right triangles and oblique triangles.
When can you use the Law of Sines to find a missing angle?
In the Law of Sines, what is the relationship between the angle in the numerator and the side in the denominator?
What type of triangle results in an ambiguous case?
For the following exercises, assumeis opposite sideis opposite sideandis opposite sideSolve each triangle, if possible. Round each answer to the nearest tenth.
For the following exercises, use the Law of Sines to solve for the missing side for each oblique triangle. Round each answer to the nearest hundredth. Assume that angleis opposite sideangleis opposite sideand angleis opposite side
Find side when
For the following exercises, assumeis opposite sideis opposite sideandis opposite sideDetermine whether there is no triangle, one triangle, or two triangles. Then solve each triangle, if possible. Round each answer to the nearest tenth.
For the following exercises, use the Law of Sines to solve, if possible, the missing side or angle for each triangle or triangles in the ambiguous case. Round each answer to the nearest tenth.
For the following exercises, find the area of the triangle with the given measurements. Round each answer to the nearest tenth.
For the following exercises, find the length of sideRound to the nearest tenth.
For the following exercises, find the measure of angleif possible. Round to the nearest tenth.
Notice thatis an obtuse angle.
For the following exercises, find the area of each triangle. Round each answer to the nearest tenth.
Find the radius of the circle in Figure 18. Round to the nearest tenth.
Findin Figure 20. Round to the nearest tenth.
Solve both triangles in Figure 22. Round each answer to the nearest tenth.
Solve the triangle in Figure 24. (Hint: Draw a perpendicular fromtoRound each answer to the nearest tenth.
In Figure 26,is not a parallelogram.is obtuse. Solve both triangles. Round each answer to the nearest tenth.
To determine how far a boat is from shore, two radar stations 500 feet apart find the angles out to the boat, as shown in Figure 28. Determine the distance of the boat from stationand the distance of the boat from shore. Round your answers to the nearest whole foot.
Figure 29 shows a satellite orbiting Earth. The satellite passes directly over two tracking stationsandwhich are 69 miles apart. When the satellite is on one side of the two stations, the angles of elevation atandare measured to beandrespectively. How far is the satellite from stationand how high is the satellite above the ground? Round answers to the nearest whole mile.
A communications tower is located at the top of a steep hill, as shown in Figure 30. The angle of inclination of the hill isA guy wire is to be attached to the top of the tower and to the ground, 165 meters downhill from the base of the tower. The angle formed by the guy wire and the hill isFind the length of the cable required for the guy wire to the nearest whole meter.
Similar to an angle of elevation, an angle of depression is the acute angle formed by a horizontal line and an observer’s line of sight to an object below the horizontal. A pilot is flying over a straight highway. He determines the angles of depression to two mileposts, 6.6 km apart, to beandas shown in Figure 32. Find the distance of the plane from pointto the nearest tenth of a kilometer.
In order to estimate the height of a building, two students stand at a certain distance from the building at street level. From this point, they find the angle of elevation from the street to the top of the building to be 39°. They then move 300 feet closer to the building and find the angle of elevation to be 50°. Assuming that the street is level, estimate the height of the building to the nearest foot.
In order to estimate the height of a building, two students stand at a certain distance from the building at street level. From this point, they find the angle of elevation from the street to the top of the building to be 35°. They then move 250 feet closer to the building and find the angle of elevation to be 53°. Assuming that the street is level, estimate the height of the building to the nearest foot.
Pointsandare on opposite sides of a lake. Pointis 97 meters fromThe measure of angleis determined to be 101°, and the measure of angleis determined to be 53°. What is the distance fromtorounded to the nearest whole meter?
A man and a woman standingmiles apart spot a hot air balloon at the same time. If the angle of elevation from the man to the balloon is 27°, and the angle of elevation from the woman to the balloon is 41°, find the altitude of the balloon to the nearest foot.
Two search teams spot a stranded climber on a mountain. The first search team is 0.5 miles from the second search team, and both teams are at an altitude of 1 mile. The angle of elevation from the first search team to the stranded climber is 15°. The angle of elevation from the second search team to the climber is 22°. What is the altitude of the climber? Round to the nearest tenth of a mile.
A street light is mounted on a pole. A 6-foot-tall man is standing on the street a short distance from the pole, casting a shadow. The angle of elevation from the tip of the man’s shadow to the top of his head of 28°. A 6-foot-tall woman is standing on the same street on the opposite side of the pole from the man. The angle of elevation from the tip of her shadow to the top of her head is 28°. If the man and woman are 20 feet apart, how far is the street light from the tip of the shadow of each person? Round the distance to the nearest tenth of a foot.
Three cities,andare located so that cityis due east of cityIf cityis located 35° west of north from cityand is 100 miles from cityand 70 miles from cityhow far is cityfrom cityRound the distance to the nearest tenth of a mile.
Two streets meet at an 80° angle. At the corner, a park is being built in the shape of a triangle. Find the area of the park if, along one road, the park measures 180 feet, and along the other road, the park measures 215 feet.
Brian’s house is on a corner lot. Find the area of the front yard if the edges measure 40 and 56 feet, as shown in Figure 34.
The Bermuda triangle is a region of the Atlantic Ocean that connects Bermuda, Florida, and Puerto Rico. Find the area of the Bermuda triangle if the distance from Florida to Bermuda is 1030 miles, the distance from Puerto Rico to Bermuda is 980 miles, and the angle created by the two distances is 62°.
A yield sign measures 30 inches on all three sides. What is the area of the sign?