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1. Watch the video from Khan Academy on "Writing the Equations of Lines" in the pink square.

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Solving a quadratic equation by factoring | Algebra II | Khan Academy

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Introduction to solving an equation with variables on both sides | Algebra I | Khan Academy

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Definition: A shape, formed by two lines or rays diverging from a common point (the vertex).

Try this Adjust the angle below by dragging the orange dot.

Attributes

Vertex

The vertex is the common point at which the two lines or rays are joined. Point B is the figure above is the vertex of the angle ∠ABC.

Legs

The legs (sides) of an angle are the two lines that make it up. In the figure above, the line segments AB and BC are the legs of the angle ∠ABC.

Interior

The interior of an angle is the space in the 'jaws' of the angle extending out to infinity. See Interior of an Angle

Exterior

All the space on the plane that is not the interior. See Interior of an Angle

Identifying an angle
An angle can be identified in two ways.

1. The angle symbol, followed by three points that define the angle, with the middle letter being the vertex, and the other two on the legs. So in the figure above the angle would be ∠ABC or ∠CBA. So long as the vertex is the middle letter, the order is not important. As a shorthand we can use the 'angle' symbol. For example '∠ABC' would be read as 'the angle ABC'.

2. Just by the vertex, so long as it is not ambiguous. So in the figure above the angle could also be called simply '∠B'

Measure of an angle
The size of an angle is measured in degrees (see Angle Measures). When we say 'the angle ABC' we mean the actual angle object. If we want to talk about the size, or measure, of the angle in degrees, we should say 'the measure of the angle ABC' - often written m∠ABC.

However, many times we will see '∠ABC=34°'. Strictly speaking this is an error. It should say 'm∠ABC=34°'
Types of angle
Altogether, there are six types of angle as listed below. Click on an image for a full description of that type and a corresponding interactive applet.

In Trigonometry

When used in trigonometry, angles have some extra properties: They can have a measure greater than 360°, can be positive and negative, and are positioned on a coordinate grid with x and y axes. They are usually measured in radians instead of degrees. For more on this see Angle definition and properties (trigonometry).

Angle construction
In the Constructions chapter, there are animated demonstrations of various constructions of angles using only a compass and straightedge.

Other angle topics

General

Angle Types

Angle relationships

(C) 2011 Copyright Math Open Reference.
All rights reserved

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Definition: A shape, formed by two lines or rays diverging from a common point (the vertex). Try this Adjust the angle below by dragging the orange dot. Vertex The vertex is the common point at which the two lines or rays are joined. Point B is the figure above is the vertex of the angle ∠ABC.

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Writing equations in all forms | Algebra I | Khan Academy

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Identifying parallel and perpendicular lines | Geometry | Khan Academy

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Perpendicular lines from equation | Mathematics I | High School Math | Khan Academy

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Coordinate plane examples | Linear equations and functions | 8th grade | Khan Academy

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Practice Graphing on the Coordinate Plane

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Coordinate plane problems in all four quadrants (practice) | Khan Academy

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Solving quadratics by factoring

Solving quadratics by factoring

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Quadratics by factoring (intro) (practice) | Khan Academy

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Equations with variables on both sides (practice) | Khan Academy

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Name angles | Angle introduction (practice) | Khan Academy

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Identify points, lines, line segments, rays, and angles (practice) | Khan Academy

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Summary: Forms of two-variable linear equations

Summary: Forms of two-variable linear equations

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Linear equations in any form | Algebra (practice) | Khan Academy

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Write equations of parallel & perpendicular lines | Analytic geometry (practice) | Khan Academy

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