# Angles and Radian Measure

9.2

Angles and Radian Measure

Essential Question How can you find the measure of an angle

in radians?

Let the vertex of an angle be at the origin, with one side of the angle on the positive x-axis. The radian measure of the angle is a measure of the intercepted arc length on a circle of radius 1. To convert between degree and radian measure, use the fact that

-- r1a8d0ia?ns = 1.

Writing Radian Measures of Angles

Work with a partner. Write the radian measure of each angle with the given degree measure. Explain your reasoning.

a. radian measure degree measure

135? 180?

225?

y

b.

90?

45?

0? 360? x

315? 270?

120? 150?

210? 240?

y

60? 30?

x

330? 300?

REASONING ABSTRACTLY

To be proficient in math, you need to make sense of quantities and their relationships in problem situations.

Writing Degree Measures of Angles

Work with a partner. Write the degree measure of each angle with the given radian measure. Explain your reasoning.

degree measure radian measure

y

5 4

99

7

2

9

9

x

11

16

9

9

13 14

99

Communicate Your Answer

3. How can you find the measure of an angle in radians?

4. The figure shows an angle whose measure is 30 radians. What is the measure of the angle in degrees? How many times greater is 30 radians than 30 degrees? Justify your answers.

y

x

30 radians

Section 9.2 Angles and Radian Measure 469

9.2 Lesson

Core Vocabulary

initial side, p. 470 terminal side, p. 470 standard position, p. 470 coterminal, p. 471 radian, p. 471 sector, p. 472 central angle, p. 472

Previous radius of a circle circumference of a circle

What You Will Learn

Draw angles in standard position. Find coterminal angles. Use radian measure.

Drawing Angles in Standard Position

In this lesson, you will expand your study of angles to include angles with measures that can be any real numbers.

Core Concept

Angles in Standard Position

In a coordinate plane, an angle can be formed by fixing one ray, called the initial side, and rotating the other ray, called the terminal side, about the vertex.

An angle is in standard position when its vertex is at the origin and its initial side lies on the positive x-axis.

90? y terminal

side

0?

180? vertex

initial side

x

360?

270?

The measure of an angle is positive when the rotation of its terminal side is counterclockwise and negative when the rotation is clockwise. The terminal side of an angle can rotate more than 360?.

Drawing Angles in Standard Position

Draw an angle with the given measure in standard position.

a. 240?

b. 500?

c. -50?

SOLUTION

a. Because 240? is 60? more than 180?, the terminal side is 60? counterclockwise past the negative x-axis.

y

240?

x

60?

b. Because 500? is 140? more than 360?, the terminal side makes one complete rotation 360? counterclockwise plus 140? more.

y

c. Because -50? is negative, the terminal side is 50? clockwise from the positive x-axis.

y

140?

x

500?

x

-50?

Monitoring Progress

Help in English and Spanish at

Draw an angle with the given measure in standard position.

1. 65?

2. 300?

3. -120?

4. -450?

470 Chapter 9 Trigonometric Ratios and Functions

STUDY TIP

If two angles differ by a multiple of 360?, then the angles are coterminal.

Finding Coterminal Angles

In Example 1(b), the angles 500? and 140? are coterminal because their terminal sides coincide. An angle coterminal with a given angle can be found by adding or subtracting multiples of 360?.

Finding Coterminal Angles

Find one positive angle and one negative angle that are coterminal with (a) -45? and (b) 395?.

SOLUTION

There are many such angles, depending on what multiple of 360? is added or subtracted.

a. -45? + 360? = 315? -45? - 360? = -405?

b. 395? - 360? = 35? 395? - 2(360?) = -325?

y

y

-325?

-45?

x

315?

-405?

35?

395?

x

STUDY TIP

Notice that 1 radian is approximately equal to 57.3?.

180? = radians

-- 180? = 1 radian

57.3? 1 radian

Monitoring Progress

Help in English and Spanish at

Find one positive angle and one negative angle that are coterminal with the given angle.

5. 80?

6. 230?

7. 740?

8. -135?

Using Radian Measure

Angles can also be measured in radians. To define a radian, consider a circle with radius r centered at the origin, as shown. One radian is the measure of an angle in standard position whose terminal side intercepts an arc of length r.

Because the circumference of a circle is 2r, there are 2 radians in a full circle. So, degree measure and radian measure are related by the equation 360? = 2 radians, or 180? = radians.

y

r r

1 radian

x

Core Concept

Converting Between Degrees and Radians

Degrees to radians

Radians to degrees

Multiply degree measure by

Multiply radian measure by

-- r1a8d0ia?ns.

-- r1a8d0ia?ns.

Section 9.2 Angles and Radian Measure 471

READING

The unit "radians" is often omitted. For instance, the bmeewasruitrtee-n -- 1si2mrpaldyiaans s-m-- 12a.y

Convert Between Degrees and Radians

Convert the degree measure to radians or the radian measure to degrees.

a. 120?

b. --- 12

SOLUTION

( ) a. 120? = 120 degrees -- 180raddegiarneses

= -- 23

( )( ) b. --- 12 = --- 12 radians -- r1a8d0ia?ns

= -15?

Concept Summary

Degree and Radian Measures of Special Angles

The diagram shows equivalent degree and radian measures for special angles from 0? to 360? (0 radians to 2 radians).

You may find it helpful to memorize the equivalent degree and radian measures of special angles in the first quadrant and for 90? = --2 radians. All other special angles shown are multiples of these angles.

2 3 3

y 2

radian measure

3

5 4

120? 90? 60?

4

6 135?

45? 6

150? degree 30?

180? measure

0? 0 x 360? 2

210?

330?

7

225?

315? 11

6 5

240?

300?

270?

6 7

4 4 3

3

2

5 4 3

Monitoring Progress

Help in English and Spanish at

Convert the degree measure to radians or the radian measure to degrees.

9. 135?

10. -40?

11. -- 54

12. -6.28

A sector is a region of a circle that is bounded by two radii and an arc of the circle. The central angle of a sector is the angle formed by the two radii. There are simple formulas for the arc length and area of a sector when the central angle is measured in radians.

Core Concept

Arc Length and Area of a Sector The arc length s and area A of a sector with radius r and central angle (measured in radians) are as follows.

Arc length: s = r

Area: A = --12 r 2

sector

r

central angle

arc length s

472 Chapter 9 Trigonometric Ratios and Functions

Modeling with Mathematics

A softball field forms a sector with the dimensions shown. Find the length of the outfield fence and the area of the field.

SOLUTION

1. Understand the Problem You are given the dimensions of a softball field. You are asked to find the length of the outfield fence and the area of the field.

outfield fence

200 ft

2. Make a Plan Find the measure of the central angle in radians. Then use the arc length and area of a sector formulas.

3. Solve the Problem

90? 200 ft

Step 1 Convert the measure of the central angle to radians.

COMMON ERROR

You must write the measure of an angle in radians when using these formulas for the arc length and area of a sector.

( ) 90? = 90 degrees -- 180raddegiarneses

= --2 radians

Step 2 Find the arc length and the area of the sector.

Arc length: s = r

Area: A = --21r 2

( ) = 200 --2

( ) = --12 (200)2 --2

ANOTHER WAY

Because the central angle is 90?, the sector represents --14 of a circle with a radius of 200 feet. So,

s = --14 2r = --41 2 (200)

= 100 314

= 10,000 31,416

The length of the outfield fence is about 314 feet. The area of the field is about 31,416 square feet.

4. Look Back To check the area of the field, consider the square formed using the two 200-foot sides.

= 100 and

A = --41 r 2 = --14 (200)2

= 10,000.

By drawing the diagonal, you can see that the area of the field is less than the area of the square but greater than one-half of the area of the square.

--12 (area of square)

area of square

--12 (200)2 ................

................

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