1. Right-Angle Trigonometry
10-1
Right-Angle Trigonometry
Warm Up
Lesson Presentation
Lesson Quiz
Holt McDougal Algebra 2
Holt Algebra 2

2. Warm Up
Given the measure of one of the acute angles in a right triangle, find the measure of the other acute angle.
1. 45° °
3. 24° °
45°
30°
66°
52°

3. Warm Up Continued
Find the unknown length for each right triangle with legs a and b and hypotenuse c.
5. b = 12, c =13
6. a = 3, b = 3
a = 5

4. Objectives
Understand and use trigonometric relationships of acute angles in triangles.
Determine side lengths of right triangles by using trigonometric functions.

5. Vocabulary trigonometric function sine cosine tangent cosecants secant
cotangent

6. A trigonometric function is a function whose rule is given by a trigonometric ratio. A trigonometric ratio compares the lengths of two sides of a right triangle. The Greek letter theta θ is traditionally used to represent the measure of an acute angle in a right triangle. The values of trigonometric ratios depend upon θ.

8. The triangle shown at right is similar to the one in the table because their corresponding angles are congruent. No matter which triangle is used, the value of sin θ is the same. The values of the sine and other trigonometric functions depend only on angle θ and not on the size of the triangle.

9. Example 1: Finding Trigonometric Ratios
Find the value of the sine, cosine, and tangent functions for θ.
sin θ =
cos θ =
tan θ =

10. Check It Out! Example 1
Find the value of the sine, cosine, and tangent functions for θ.
sin θ =
cos θ =
tan θ =

11. You will frequently need to determine the value of trigonometric ratios for 30°,60°, and 45° angles as you solve trigonometry problems. Recall from geometry that in a 30°-60°-90° triangle, the ration of the side lengths is 1: 3 :2, and that in a 45°-45°-90° triangle, the ratio of the side lengths is 1:1: 2.

13. Example 2: Finding Side Lengths of Special Right Triangles
Use a trigonometric function to find the value of x.
The sine function relates the opposite leg and the hypotenuse.
Substitute 30° for θ, x for opp, and 74 for hyp.
Substitute for sin 30°.
x = 37
Multiply both sides by 74 to solve for x.

14. Check It Out! Example 2
Use a trigonometric function to find the value of x.
The sine function relates the opposite leg and the hypotenuse.
Substitute 45 for θ, x for opp, and 20 for hyp.
Substitute for sin 45°.
Multiply both sides by 20 to solve for x.

15. Example 3: Sports Application
In a waterskiing competition, a jump ramp has the measurements shown. To the nearest foot, what is the height h above water that a skier leaves the ramp?
Substitute 15.1° for θ, h for opp., and 19 for hyp.
Multiply both sides by 19.
Use a calculator to simplify.
5 ≈ h
The height above the water is about 5 ft.

16. Make sure that your graphing calculator is set to interpret angle values as degrees. Press Check that Degree and not Radian is highlighted in the third row.
Caution!

17. Check It Out! Example 3
A skateboard ramp will have a height of 12 in., and the angle between the ramp and the ground will be 17°. To the nearest inch, what will be the length l of the ramp?
Substitute 17° for θ, l for hyp., and 12 for opp.
Multiply both sides by l and divide by sin 17°.
l ≈ 41
Use a calculator to simplify.
The length of the ramp is about 41 in.

18. When an object is above or below another object, you can find distances indirectly by using the angle of elevation or the angle of depression between the objects.

19. Example 4: Geology Application
A biologist whose eye level is 6 ft above the ground measures the angle of elevation to the top of a tree to be 38.7°. If the biologist is standing 180 ft from the tree’s base, what is the height of the tree to the nearest foot?
Step 1 Draw and label a diagram to represent the information given in the problem.

20. Example 4 Continued
Step 2 Let x represent the height of the tree compared with the biologist’s eye level. Determine the value of x.
Use the tangent function.
Substitute 38.7 for θ, x for opp., and 180 for adj.
180(tan 38.7°) = x
Multiply both sides by 180.
144 ≈ x
Use a calculator to solve for x.

21. Example 4 Continued
Step 3 Determine the overall height of the tree.
x + 6 =
= 150
The height of the tree is about 150 ft.

22. Check It Out! Example 4
A surveyor whose eye level is 6 ft above the ground measures the angle of elevation to the top of the highest hill on a roller coaster to be 60.7°. If the surveyor is standing 120 ft from the hill’s base, what is the height of the hill to the nearest foot?
Step 1 Draw and label a diagram to represent the information given in the problem.
120 ft
60.7°

23. Check It Out! Example 4 Continued
Step 2 Let x represent the height of the hill compared with the surveyor’s eye level. Determine the value of x.
Use the tangent function.
Substitute 60.7 for θ, x for opp., and 120 for adj.
120(tan 60.7°) = x
Multiply both sides by 120.
214 ≈ x
Use a calculator to solve for x.

24. Check It Out! Example 4 Continued
Step 3 Determine the overall height of the roller coaster hill.
x + 6 =
= 220
The height of the hill is about 220 ft.

25. The reciprocals of the sine, cosine, and tangent ratios are also trigonometric ratios. They are trigonometric functions, cosecant, secant, and cotangent.

26. Example 5: Finding All Trigonometric Functions
Find the values of the six trigonometric functions for θ.
Step 1 Find the length of the hypotenuse. 70 24 θ
a2 + b2 = c2
Pythagorean Theorem.
Substitute 24 for a and 70 for b.
c2 =
c2 = 5476
Simplify.
Solve for c. Eliminate the negative solution.
c = 74

27. Example 5 Continued
Step 2 Find the function values.

28. In each reciprocal pair of trigonometric functions, there is exactly one “co”
Helpful Hint

29. Find the values of the six trigonometric functions for θ.
Check It Out! Example 5
Find the values of the six trigonometric functions for θ.
Step 1 Find the length of the hypotenuse. 80 18 θ
a2 + b2 = c2
Pythagorean Theorem.
Substitute 18 for a and 80 for b.
c2 =
c2 = 6724
Simplify.
Solve for c. Eliminate the negative solution.
c = 82

30. Check It Out! Example 5 Continued
Step 2 Find the function values.

31. Lesson Quiz: Part I
Solve each equation. Check your answer.
1. Find the values of the six trigonometric functions for θ.

32. Lesson Quiz: Part II
2. Use a trigonometric function to find the value of x.
3. A helicopter’s altitude is 4500 ft, and a plane’s altitude is 12,000 ft. If the angle of depression from the plane to the helicopter is 27.6°, what is the distance between the two, to the nearest hundred feet?
16,200 ft