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Trigonometry
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2. Copyright © Cengage Learning. All rights reserved.
1.3
RIGHT TRIANGLE TRIGONOMETRY
Copyright © Cengage Learning. All rights reserved.

3. What You Should Learn
Evaluate trigonometric functions of acute angles.
Use fundamental trigonometric identities.
Use a calculator to evaluate trigonometric functions.
Use trigonometric functions to model and solve real-life problems.

4. The Six Trigonometric Functions

5. The Six Trigonometric Functions
Our second look at the trigonometric functions is from a right triangle perspective.
Consider a right triangle, with one acute angle labeled , as shown in Figure 1.26.
Figure 1.26

6. The Six Trigonometric Functions
Relative to the angle , the three sides of the triangle are the hypotenuse, the opposite side (the side opposite the angle  ), and the adjacent side (the side adjacent to the angle  ).
Using the lengths of these three sides, you can form six ratios that define the six trigonometric functions of the acute angle .
sine cosecant cosine secant tangent cotangent

7. The Six Trigonometric Functions
In the following definitions, it is important to see that 0 <  < 90 ( lies in the first quadrant) and that for such angles the value of each trigonometric function is positive.

8. Example 1 – Evaluating Trigonometric Functions
Use the triangle in Figure 1.27 to find the values of the six trigonometric functions of .
Solution:
By the Pythagorean Theorem,
(hyp)2 = (opp)2 + (adj)2, it follows that
Figure 1.27

9. Example 1 – Solution So, the six trigonometric functions of  are
cont’d
So, the six trigonometric functions of  are

10. The Six Trigonometric Functions
In Example 1, you were given the lengths of two sides of the right triangle, but not the angle .
Often, you will be asked to find the trigonometric functions of a given acute angle . To do this, construct a right triangle having  as one of its angles.

11. The Six Trigonometric Functions
In the box, note that sin 30 = = cos 60. This occurs because 30 and 60 are complementary angles.
In general, it can be shown from the right triangle definitions that cofunctions of complementary angles are equal. That is, if  is an acute angle, the following relationships are true.
sin(90 –  ) = cos  cos(90 –  ) = sin 
tan(90 –  ) = cot  cot(90 –  ) = tan 
sec(90 –  ) = csc  csc(90 –  ) = sec 

12. Trigonometric Identities

13. Trigonometric Identities
In trigonometry, a great deal of time is spent studying relationships between trigonometric functions (identities).
Note that sin2  represents (sin  )2, cos2  represents (cos  )2, and so on.

14. Example 4 – Applying Trigonometric Identities
Let  be an acute angle such that sin  = 0.6. Find the values of (a) cos  and (b) tan  using trigonometric identities.
Solution:
a. To find the value of cos , use the Pythagorean identity
sin2  + cos2  = 1.
So, you have
(0.6)2 + cos2  = 1
cos2  = 1 – (0.6)2
= 0.64
Substitute 0.6 for sin .
Subtract (0.6)2 from each side.

15. Example 4 – Solution cos  = = 0.8.
cont’d
cos  =
= 0.8.
b. Now, knowing the sine and cosine of , you can find the tangent of  to be
Use the definitions of cos  and tan , and the triangle shown in Figure 1.30, to check these results.
Extract the positive square root.
= 0.75.
Figure 1.30

16. Evaluating Trigonometric Functions with a Calculator

17. Evaluating Trigonometric Functions with a Calculator
To use a calculator to evaluate trigonometric functions of angles measured in degrees, first set the calculator to degree mode and then proceed.
For instance, you can find values of cos 28 and sec 28 as follows.
Function Mode Calculator Keystrokes Display
a. cos 28 Degree
b. sec 28 Degree

18. Evaluating Trigonometric Functions with a Calculator
Throughout this text, angles are assumed to be measured in radians unless noted otherwise.
For example, sin 1 means the sine of 1 radian and sin 1 means the sine of 1 degree.

19. Example 6 – Using a Calculator
Use a calculator to evaluate sec(5 40 12).
Solution:
Begin by converting to decimal degree form.
[Recall that ].
5 40 12 =
= 5.67

20. Example 6 – Solution Then, use a calculator to evaluate sec 5.67.
cont’d
Then, use a calculator to evaluate sec 5.67.
Function Calculator Keystrokes Display

21. Applications Involving Right Triangles

22. Applications Involving Right Triangles
Many applications of trigonometry involve a process called solving right triangles.
In this type of application, you are usually given one side of a right triangle and one of the acute angles and are asked to find one of the other sides, or you are given two sides and are asked to find one of the acute angles.
In Example 7, the angle you are given is the angle of elevation, which represents the angle from the horizontal upward to an object.

23. Applications Involving Right Triangles
For objects that lie below the horizontal, it is common to use the term angle of depression, as shown in Figure 1.32.
Figure 1.32

24. Example 7 – Using Trigonometry to Solve a Right Triangle
A surveyor is standing 115 feet from the base of the Washington Monument, as shown in Figure The surveyor measures the angle of elevation to the top of the monument as 78.3. How tall is the Washington Monument?
Figure 1.33

25. Example 7 – Solution From Figure 1.33, you can see that
where x = 115 and y is the height of the monument.
So, the height of the Washington Monument is
y = x tan 78.3
 115( )
 555 feet.

26. Applications Involving Right Triangles
By now you are able to recognize that  = 30 is the acute angle that satisfies the equation sin  =
Suppose, however, that you were given the equation sin  = 0.6 and were asked to find the acute angle . Because
and
you might guess that  lies somewhere between 30 and 45.