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Alternate Interior Angles
See It in Action
Two examples of the idea done right—and one common look-alike that is not right.
EXAMPLE
EXAMPLE
NON-EXAMPLE
Definition
Alternate interior angles are a pair of angles formed when a transversal intersects two lines, and they are:
On opposite sides of the transversal
Between (or inside) the two lines
Key fact:
If the two lines are parallel, then alternate interior angles are congruent (equal in measure).
Example:
If line l and line m are parallel, and transversal t crosses them, then:
Angle 3 and angle 4 form a pair of alternate interior angles, and angle 3 ≅ angle 4.
Video
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