1. Geometry Mrs. Spitz Spring 2005
9.5 Trigonometric Ratios
Geometry
Mrs. Spitz
Spring 2005

2. Objectives/Assignment
Find the since, the cosine, and the tangent of an acute triangle.
Use trionometric ratios to solve real-life problems, such as estimating the height of a tree or flagpole.
To solve real-life problems such as in finding the height of a water slide.
Assignment: pp #3-38 all
Due TODAY– 9.4
Quiz next time we meet

3. Finding Trig Ratios
A trigonometric ratio is a ratio of the lengths of two sides of a right triangle. The word trigonometry is derived from the ancient Greek language and means measurement of triangles. The three basic trigonometric ratios are sine, cosine, and tangent, which are abbreviated as sin, cos, and tan respectively.

4. Trigonometric Ratios
Let ∆ABC be a right triangle. The since, the cosine, and the tangent of the acute angle A are defined as follows.
Side adjacent to A b
cos A = =
hypotenuse c
Side opposite A a
sin A = =
hypotenuse c
Side opposite A a
tan A = =
Side adjacent to A b

5. Note:
The value of a trigonometric ratio depends only on the measure of the acute angle, not on the particular right triangle that is used to compute the value.

6. Ex. 1: Finding Trig Ratios
Compare the sine, the cosine, and the tangent ratios for A in each triangle beside.
By the SSS Similarity Theorem, the triangles are similar. Their corresponding sides are in proportion which implies that the trigonometric ratios for A in each triangle are the same.

7. Ex. 1: Finding Trig Ratios
Large
Small
opposite 8
sin A = ≈
0.4706 4 ≈
0.4706
hypotenuse 17
8.5
adjacent
7.5
cosA = 15 ≈
0.8824 ≈
0.8824
hypotenuse
8.5 17
opposite
tanA = 8 4 ≈
0.5333 ≈
0.5333
adjacent 15
7.5
Trig ratios are often expressed as decimal approximations.

8. Ex. 2: Finding Trig Ratios
opposite 5
sin S = ≈
0.3846
hypotenuse 13
adjacent
cosS = 12 ≈
0.9231
hypotenuse 13
opposite
tanS = 5 ≈
0.4167
adjacent 12

9. Ex. 2: Finding Trig Ratios—Find the sine, the cosine, and the tangent of the indicated angle.
opposite 12
sin S = ≈
0.9231
hypotenuse 13
adjacent
cosS = 5 ≈
0.3846
hypotenuse 13
opposite
tanS = 12 ≈
2.4
adjacent 5

10. Ex. 3: Finding Trig Ratios—Find the sine, the cosine, and the tangent of 45
opposite 1 √2
sin 45= = ≈
0.7071
hypotenuse √2 2
adjacent 1 √2
cos 45= = ≈
0.7071
hypotenuse √2 2
opposite 1
tan 45=
adjacent = 1 1
Begin by sketching a 45-45-90 triangle. Because all such triangles are similar, you can make calculations simple by choosing 1 as the length of each leg. From Theorem 9.8 on page 551, it follows that the length of the hypotenuse is √2. √2
45

11. Ex. 4: Finding Trig Ratios—Find the sine, the cosine, and the tangent of 30
opposite 1
sin 30= =
0.5
hypotenuse 2
adjacent √3
cos 30= ≈
0.8660
hypotenuse 2
opposite 1 √3
tan 30= =
adjacent ≈
0.5774 √3 3
Begin by sketching a 30-60-90 triangle. To make the calculations simple, you can choose 1 as the length of the shorter leg. From Theorem 9.9, on page 551, it follows that the length of the longer leg is √3 and the length of the hypotenuse is 2.
30 √3

12. Ex: 5 Using a Calculator
You can use a calculator to approximate the sine, cosine, and the tangent of 74. Make sure that your calculator is in degree mode. The table shows some sample keystroke sequences accepted by most calculators.

13. Sample keystrokes
Sample keystroke sequences
Sample calculator display
Rounded
Approximation 74
0.9613
0.2756
3.4874
sin
sin
ENTER 74
COS
COS
ENTER 74
TAN
TAN
ENTER

14. Notes:
If you look back at Examples 1-5, you will notice that the sine or the cosine of an acute triangles is always less than 1. The reason is that these trigonometric ratios involve the ratio of a leg of a right triangle to the hypotenuse. The length of a leg or a right triangle is always less than the length of its hypotenuse, so the ratio of these lengths is always less than one.

15. Trigonometric Identities
A trigonometric identity is an equation involving trigonometric ratios that is true for all acute triangles. You are asked to prove the following identities in Exercises 47 and 52.
(sin A)2 + (cos A)2 = 1
sin A
tan A =
cos A

16. Using Trigonometric Ratios in Real-life
Suppose you stand and look up at a point in the distance. Maybe you are looking up at the top of a tree as in Example 6. The angle that your line of sight makes with a line drawn horizontally is called angle of elevation.

17. Ex. 6: Indirect Measurement
You are measuring the height of a Sitka spruce tree in Alaska. You stand 45 feet from the base of the tree. You measure the angle of elevation from a point on the ground to the top of the top of the tree to be 59°. To estimate the height of the tree, you can write a trigonometric ratio that involves the height h and the known length of 45 feet.

18. The math The tree is about 76 feet tall. Write the ratio
tan 59° =
opposite
adjacent
Write the ratio
tan 59° = h 45
Substitute values
Multiply each side by 45
45 tan 59° = h
Use a calculator or table to find tan 59°
45 (1.6643) ≈ h
Simplify
75.9 ≈ h
The tree is about 76 feet tall.

19. Ex. 7: Estimating Distance
Escalators. The escalator at the Wilshire/Vermont Metro Rail Station in Los Angeles rises 76 feet at a 30° angle. To find the distance d a person travels on the escalator stairs, you can write a trigonometric ratio that involves the hypotenuse and the known leg of 76 feet.
30°

20. Now the math
sin 30° =
opposite
hypotenuse
30°
Write the ratio for sine of 30°
sin 30° = 76 d
Substitute values.
d sin 30° = 76
Multiply each side by d.
sin 30° 76
d =
Divide each side by sin 30°
0.5 76
d =
Substitute 0.5 for sin 30°
d = 152
Simplify
A person travels 152 feet on the escalator stairs.

21. Reminders: After this section, you have a quiz on Thursday or Friday.
Chapter 9 exam will take place before you leave for spring break Take it before you go on break.
Binder check before spring break. These are your new grades for the last quarter of the year. Study and don’t slack off now.