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Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians12-3-EXT Measuring Angles in Radians
Holt Geometry
Lesson PresentationLesson Presentation
Holt McDougal Geometry
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Use proportions to convert angle measures from degrees to radians.
Objectives
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
radian
Vocabulary
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
One unit of measurement for angles is degrees, which are based on a fraction of a circle. Another unit is called a radian, which is based on the relationship of the radius and arc length of a central angle in a circle.
Four concentric circles are shown, with radius 1, 2, 3, and 4. The measure of each arc is 60°.
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
The relationship between the radius and arc length is
linear, with a slope of 2π = , or about
1.05. The slope represents the ratio of the arc length to
the radius. This ratio is the radian measure of the angle,
so 60° is the same as radians.
60°360°
π3
π3
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
If a central angle θ in a circle of radius r intercepts an arc of length r, the measure of θ is defined as 1 radian. Since the circumference of a circle of radius r is 2πr, an angle representing one complete rotation measures 2π radians, or 360°.
2π radians = 360° and π radians = 180°
1° =π radians
180°and 1 radian =
180°π radians
Use these facts to convert between radians and degrees.
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Arc length is the distance along an arc measured in linear
units. In a circle of radius r, the length of an arc with a
central angle measure m is L = 2πr
Remember!
m°360°
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Convert each measure from degrees to radians.
Example 1: Converting Degrees to Radians
A. 85°
17 85°π radians
180° 36
= 17 π36
A. 90°
1 90°π radians
180° 2
= π2
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Because the radian measure of an angle is related to arc length, the most commonly used angle measures are usually written as fractional multiples of π.
Helpful Hint
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Check It Out! Example 1
Convert each measure from degrees to radians.
A. –36°
-1 -36°π radians
180° 5
= - π5
B. 270°
3 270°π radians
180° 2= 3π
2
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Convert each measure from radians to degrees.
Example 2: Converting Radians to Degrees
2π3
A.
2 π
1 3radians
60 180°
Π radians= 120°
π6
B.
π
1 6radians
30 180°
Π radians= 30°
Holt McDougal Geometry
12-3-EXT Measuring Angles in Radians
Check It Out! Example 2
Convert each measure from radians to degrees.
5π6
A.
5 π
1 6radians
30 180°
Π radians= 150°
3π4
B. -
3 π
1 4radians
45 180°
Π radians= -135° -