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Alternate Exterior Angles

Examples & Non-Examples

Example

Example

Non-Example

Geometry diagram of two parallel lines cut by a transversal, with two 60° angle labels. One labeled angle is in the top right, and the other is in the bottom left.
Geometry diagram of two parallel lines cut by a transversal, with two 120° angle labels. One labeled angle is in the top left, and the other is in the bottom right.
Geometry diagram of two parallel lines cut by a transversal, with two 120° angle labels. One labeled angle is in the inside top right, and the other is in the inside bottom left.

Definition

Alternate exterior angles are a pair of angles formed when a transversal crosses two lines, and they are:

  • On opposite sides of the transversal

  • Outside the two lines

Key fact:

If the two lines are parallel, then alternate exterior angles are congruent (equal in measure).

Example:

If line l and line m are parallel, and transversal t crosses them, then angle 1 and angle 2 form a pair of alternate exterior angles — and they are congruent.

Geometry diagram of parallel lines l and m cut by transversal t, with blue angles 1 and 2 marked at intersections. Angle 1 is in the top right, and angle 2 is in the bottom left.

Video


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