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Alternate Interior Angles Theorem
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
The Alternate Interior Angles Theorem states:
“If two parallel lines are cut by a transversal, then each pair of alternate interior angles is congruent.”
In simple terms:
When a transversal crosses two parallel lines,
The alternate interior angles (on opposite sides of the transversal and between the two lines)
Are always equal in measure.
Example:
If line l is parallel to line m, and transversal t intersects them, then:
Angle 5 and angle 6 form a pair of alternate interior angles, and angle 5 ≅ angle 6.
Video
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