1. 3 Radian Measure and Circular Functions
Copyright © 2009 Pearson Addison-Wesley

2. Radian Measure and Circular Functions3
3.1 Radian Measure
3.2 Applications of Radian Measure
3.3 The Unit Circle and Circular Functions
3.4 Linear and Angular Speed
Copyright © 2009 Pearson Addison-Wesley

3. The Unit Circle and Circular Functions
3.3
The Unit Circle and Circular Functions
Circular Functions ▪ Finding Values of Circular Functions ▪ Determining a Number with a Given Circular Function Value ▪ Applying Circular Functions
Copyright © 2009 Pearson Addison-Wesley 1.1-3

4. Circular Functions
A unit circle has its center at the origin and a radius of 1 unit.
The trigonometric functions of angle θ in radians are found by choosing a point (x, y) on the unit circle can be rewritten as functions of the arc length s.
When interpreted this way, they are called circular functions.
Copyright © 2009 Pearson Addison-Wesley

5. Circular Functions
For any real number s represented by a directed arc on the unit circle,
Copyright © 2009 Pearson Addison-Wesley 1.1-5

6. The Unit Circle
Copyright © 2009 Pearson Addison-Wesley

7. The Unit Circle
The unit circle is symmetric with respect to the x-axis, the y-axis, and the origin.
If a point (a, b) lies on the unit circle, so do (a,–b), (–a, b) and (–a, –b).
Copyright © 2009 Pearson Addison-Wesley

8. The Unit Circle
For a point on the unit circle, its reference arc is the shortest arc from the point itself to the nearest point on the x-axis.
For example, the quadrant I real number
is associated with the point on the unit circle.
Copyright © 2009 Pearson Addison-Wesley

9. Copyright © 2009 Pearson Addison-Wesley 1.1-9

10. The Unit Circle
Since sin s = y and cos s = x, we can replace x and y in the equation of the unit circle
to obtain the Pythagorean identity
Copyright © 2009 Pearson Addison-Wesley

11. Domains of Circular Functions
Sine and Cosine Functions:
Tangent and Secant Functions:
Cotangent and Cosecant Functions:
Copyright © 2009 Pearson Addison-Wesley

12. Evaluating A Circular Function
Circular function values of real numbers are obtained in the same manner as trigonometric function values of angles measured in radians.
This applies both to methods of finding exact values (such as reference angle analysis) and to calculator approximations.
Calculators must be in radian mode when finding circular function values.
Copyright © 2009 Pearson Addison-Wesley

13. Find the exact values of
Example 1
FINDING EXACT CIRCULAR FUNCTION VALUES
Find the exact values of
Evaluating a circular function at the real number is equivalent to evaluating it at radians.
An angle of intersects the circle at the point (0, –1).
Since sin s = y, cos s = x, and
Copyright © 2009 Pearson Addison-Wesley

14. Use the figure to find the exact values of
Example 2(a)
FINDING EXACT CIRCULAR FUNCTION VALUES
Use the figure to find the exact values of
The real number
corresponds to the unit circle point
Copyright © 2009 Pearson Addison-Wesley

15. yields the same ending point as moving around the
Example 2(b)
FINDING EXACT CIRCULAR FUNCTION VALUES
Use the figure and the definition of tangent to find the exact value of
negative direction
yields the same ending point as moving around the
Moving around the unit circle units in the
circle units in the positive direction.
Copyright © 2009 Pearson Addison-Wesley

16. Example 2(b) corresponds to FINDING EXACT CIRCULAR FUNCTION VALUES
Copyright © 2009 Pearson Addison-Wesley

17. An angle of corresponds to an angle of 120°.
Example 2(c)
FINDING EXACT CIRCULAR FUNCTION VALUES
Use reference angles and degree/radian conversion to find the exact value of
An angle of corresponds to an angle of 120°.
In standard position, 120° lies in quadrant II with a reference angle of 60°, so
Cosine is negative in quadrant II.
Copyright © 2009 Pearson Addison-Wesley

18. Find a calculator approximation for each circular function value.
Example 3
APPROXIMATING CIRCULAR FUNCTION VALUES
Find a calculator approximation for each circular function value.
(a) cos 1.85
≈ –.2756
(b) cos .5149
≈ .8703
Copyright © 2009 Pearson Addison-Wesley

19. Find a calculator approximation for each circular function value.
Example 3
APPROXIMATING CIRCULAR FUNCTION VALUES (continued)
Find a calculator approximation for each circular function value.
(c) cot
≈ .2552
(d) sec –2.9234
≈ –1.0243
Copyright © 2009 Pearson Addison-Wesley

20. Caution
A common error in trigonometry is using a calculator in degree mode when radian mode should be used.
Remember, if you are finding a circular function value of a real number, the calculator must be in radian mode.
Copyright © 2009 Pearson Addison-Wesley

21. Use the inverse cosine function of a calculator.
Example 4(a)
FINDING A NUMBER GIVEN ITS CIRCULAR FUNCTION VALUE
Approximate the value of s in the interval if cos s =
Use the inverse cosine function of a calculator.
, so in the given interval, s ≈
Copyright © 2009 Pearson Addison-Wesley

22. Find the exact value of s in the interval if tan s = 1.
Example 4(b)
FINDING A NUMBER GIVEN ITS CIRCULAR FUNCTION VALUE
Find the exact value of s in the interval if tan s = 1.
Recall that , and in quadrant III, tan s is negative.
Copyright © 2009 Pearson Addison-Wesley

23. Example 5
MODELING THE ANGLE OF ELEVATION OF THE SUN
The angle of elevation of the sun in the sky at any latitude L is calculated with the formula
where corresponds to sunrise and occurs if the sun is directly overhead. ω is the number of radians that Earth has rotated through since noon, when ω = 0. D is the declination of the sun, which varies because Earth is tilted on its axis.
Copyright © 2009 Pearson Addison-Wesley

24. Example 5
MODELING THE ANGLE OF ELEVATION OF THE SUN (continued)
Sacramento, CA has latitude L = 38.5° or radian. Find the angle of elevation of the sun θ at 3 P.M. on February 29, 2008, where at that time, D ≈ –.1425 and ω ≈
Copyright © 2009 Pearson Addison-Wesley

25. The angle of elevation of the sun is about .4773 radian or 27.3°.
Example 5
MODELING THE ANGLE OF ELEVATION OF THE SUN (continued)
The angle of elevation of the sun is about radian or 27.3°.
Copyright © 2009 Pearson Addison-Wesley