Angle

Pronunciation: /ˈæŋ gəl/ ?

An angle is the rotation between two intersecting lines, rays or line segments[1].

Angles defined by lines.
Figure 1: Various angles

The vertex of an angle is the point of intersection of the lines.[3 page 9] In figure 1, the vertices are the points b, h, and i. The legs, also called sides of an angle are the lines, rays, or line segments that define the angle.[3 page 9]

Two angles are equiangular if the measures of the angles are the same.

Article Index

Vertex of an Angle
legs of an Angle
Equiangular
Measure of an Angle
empty spaceDegree
Radian
Gradian
Classes of Angles
Acute Angle
Right Angle
Obtuse Angle
Straight Angle
Reflex Angle
Full Angle
Inscribed Angle
Central Angle
Angle of Rotation
Complementary Angle
Supplementary Angle
Copy an Angle
Angle Bisector
Bisect an Angle
Angle Addition Postulate

Measure of an Angle

The measure of an angle is made in terms of the measure of a full circle. The unit of measure for an angle is degrees, radians or, in rare cases, gradians.

Degree

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Manipulative 1: Degree angle. Created with GeoGebra.
One degree is 1/360 of a circle.[3 page 10] Degree is the oldest unit of measure for an angle. Degrees are denoted by a small circle (° ) or the abbreviation deg. By definition, a full circle is 360°. This means that an angle that is 1/4 of a circle is 360°/4, or 90°. See manipulative 1. Click on the blue point in manipulative 1 and drag it to change the figure.

check mark Understanding Check

Calculate the answer to the problem and write it down. Then click on the blue and yellow words to see the correct answer.

  1. The degree measure of an angle that is 1/3 of a circle is: Click for answer.360 / 3 = 120
  2. The degree measure of an angle that is 1/17 of a circle is: Click for answer.360 / 17 ≈ 21.17

Radian

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Manipulative 2: Angle in radians. Created with GeoGebra.
When measuring angles, the radian is particularly useful. A radian is defined as the angle made with an arc length of 1 on a unit circle. This means that the length of an arc of 1 radian is the same as the length of the radius of the circle. See manipulative 2. There are 2π radians in a full circle. An angle that is 1/5 of a circle is 2π/5 ≈ 1.26 radians. The abbreviation for radian is rad. Click on the blue point in manipulative 1 and drag it to change the figure.

One reason the measure of radians is so useful has to do with Euler's famous equation that relates exponentiation with trigonometry using complex numbers: e = cos(θ) + i·sin(θ). This equation only works if the angle θ is measured in radians.

check mark Understanding Check

Calculate the answer to the problem and write it down. Then click on the blue and yellow words to see the correct answer.

  1. The radian measure of an angle that is 1/3 of a circle is: Click for answer.2·π / 3 ≈ 2.09
  2. The radian measure of an angle that is 1/17 of a circle is: Click for answer.2·π / 17 ≈ 0.37

Gradian

A rarely used angle measure is gradians. A full circle measures 400 gradians. The abbreviation for gradians is grad.

Classes of Angles

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Manipulative 3: Angle classes. Created with GeoGebra.

For convenience in discussions of angles and trigonometry, angles are divided into classes.[3 page 10] The class an angle belongs to is determined by its measure. Table 1 shows the classes of angles and their measures. Click on the blue point in manipulative 3 and drag it to change the figure. Manipulative 3 also shows the classes of angles and their measures.

Table 1: Classes of Angles
Ex.Angle MeasureClass
DegreesRadians
example of an acute angle0 < θ < 900 < θ < π/2Acute angle
θ = 90θ = π/2Right angle
example of an obtuse angle90 < θ < 180π/2 < θ < πObtuse angle
example of a straight angleθ = 180θ = πStraight angle
example of a reflex angle180 < θ < 360π < θ < 2·πReflex angle
example of a full angleθ = 360θ = 2·πFull angle

Inscribed Angle

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Manipulative 4: Inscribed angle. Created with GeoGebra.
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Manipulative 5: Angle inscribed in the diameter of a circle. Created with GeoGebra.
An inscribed angle is an angle drawn inside a circle.

Discovery

  1. All inscribed angles are greater than what measure?
    Click for Answer
  2. All inscribed angles are smaller than what measure?
    Click for Answer
  3. How does the measure of the inscribed angle change when only the vertex is moved without moving across one of the endpoints?
    Click for Answer.
  4. How does the measure of the inscribed angle change when the vertex is moved across one of the endpoints?
    Click for Answer.
  5. If an angle is inscribed in the diameter of a circle, what is the measure of the inscribed angle?
    Click for Answer.

Central Angle

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Manipulative 6: Central angle of a circle. Created with GeoGebra.

A central angle of a circle is an angle with the vertex at the center of the circle and the other two points on the circumference of the circle.[3 page 41] Click on the blue points in manipulative 6 and drag them to change the figure.

For an inscribed angle and a central angle with the same endpoints, the measure of the inscribed angle is half the measure of the central angle. Click on the blue points in manipulative 7 and drag them to change the figure.

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Manipulative 7: Relationship between central and inscribed angles. Created with GeoGebra.

Angle of Rotation

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Manipulative 8: Angle of rotation. Created with GeoGebra.

An angle of rotation is the amount of rotation about a center of rotation. Click on the blue points in manipulative 8 and drag them to change the figure.

Complementary Angles

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Manipulative 9: Complementary Angles. Created with GeoGebra.
Two angles are complementary if they produce a right angle when combined.[3, page 3] This means that given angles α and β, α and β are complementary if α + β = π/2.

Supplementary Angles

Two angles are supplementary if they produce a straight line when combined.[3, page 3] This means that given angles α and β, α and β are supplementary if α + β = π.
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Manipulative 10: Supplementary Angles. Created with GeoGebra.

Copying an Angle

An angle can be copied using a compass and a straight edge.
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Manipulative 11: Copying an angle. Created with GeoGebra.

Angle Bisector

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Manipulative 12: Bisecting an angle. Created with GeoGebra.
An angle bisector is a line segment or ray that divides an angle into two congruent angles. For details on how to bisect an angle, see How to Bisect an Angle.

Angle Addition Postulate

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Manipulative 13: Angle addition postulate. Click on the blue points and drag them to change the figure.

The angle addition postulate states that adjacent angles can be added together to form a larger angle. This is a postulate or axiom, meaning it is accepted as true without proof. The exact mathematical definition of the Angle Addition Postulate is:

Given noncollinear points A, B, C and a point D in the interior of ∠BAC, m∠BAD + m∠DAC = m∠BAC.

Table of Figures

  1. Various angles
  2. 90 degree angle
  3. 1 radian
  4. Inscribed angles
  5. An angle inscribed in a circle
  6. An angle inscribed in the diameter of a circle
  7. Central angle of a circle.
  8. Relationship between central and inscribed angles
  9. Complementary Angles
  10. Supplementary Angles
  11. Copying an Angle
  12. Angle Bisector
  13. Bisecting an Angle

Educator Resources

Cite this article as:


Angle. 2009-12-26. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/a/angle.html.

Translations

Image Credits

Revision History


2009-12-26: Added "References" (McAdams, David.)
2008-10-28: Changed manipulatives and some graphics to geogebra (McAdams, David.)
2008-09-19: Changed heading 'Other Information' to 'More Information', dicitonary.com to more information (McAdams, David.)
2008-07-07: Corrected link errors (McAdams, David.)
2008-04-28: Added keyword class to angle classifications (McAdams, David.)
2008-04-19: Revised bisecting an angle table to reflect most common method (McAdams, David.)
2008-03-11: Fixed various formatting and link errors (McAdams, David.)
2008-02-03: Changed HTML entity angle to the word angle (McAdams, David.)
2007-08-17: Initial version (McAdams, David.)

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