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

Radian Measure and Circular Functions 3 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

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

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

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

The Unit Circle Copyright © 2009 Pearson Addison-Wesley

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

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

Copyright © 2009 Pearson Addison-Wesley 1.1-9

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

Domains of Circular Functions Sine and Cosine Functions: Tangent and Secant Functions: Cotangent and Cosecant Functions: Copyright © 2009 Pearson Addison-Wesley 1.1-11

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 1.1-12

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 1.1-13

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 1.1-14

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 1.1-15

Example 2(b) corresponds to FINDING EXACT CIRCULAR FUNCTION VALUES Copyright © 2009 Pearson Addison-Wesley 1.1-16

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 1.1-17

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 1.1-18

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 1.3209 ≈ .2552 (d) sec –2.9234 ≈ –1.0243 Copyright © 2009 Pearson Addison-Wesley 1.1-19

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 1.1-20

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 = .9685. Use the inverse cosine function of a calculator. , so in the given interval, s ≈ .2517. Copyright © 2009 Pearson Addison-Wesley 1.1-21

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 1.1-22

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 1.1-23

Example 5 MODELING THE ANGLE OF ELEVATION OF THE SUN (continued) Sacramento, CA has latitude L = 38.5° or .6720 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 ≈ .7854. Copyright © 2009 Pearson Addison-Wesley 1.1-24

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 .4773 radian or 27.3°. Copyright © 2009 Pearson Addison-Wesley 1.1-25