What are alternate interior angles?

1 Answer
Zor Shekhtman
Feb 5, 2016

Two parallel lines and one transversal intersecting it form #8# different angles.
Two pairs of these angles are called alternate interior.
See picture below.

Explanation:

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A pair of angles #a# and #d# are alternate interior.
So is a pair of angles #b# and #c#.

They are called alternate because #a# and #d# (as well as #b# and #c#) lie on opposite sides from transversal #l#.
They are called interior because both #a# and #d# (as well as #b# and #c#) are inside a space between the parallel lines.

Two angles forming an alternate interior pair are always congruent to each other, they have exactly the same measure.

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