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Alternate Interior Angles - Concept 16,384 views

Teacher/Instructor Brian McCall
Brian McCall

Univ. of Wisconsin
J.D. Univ. of Wisconsin Law school

Brian was a geometry teacher through the Teach for America program and started the geometry program at his school

Alternate interior angles are formed by a transversal intersecting two parallel lines . They are located between the two parallel lines but on opposite sides of the transversal, creating two pairs (four total angles) of alternate interior angles. Alternate interior angles are congruent, meaning they have equal measure.

When we have two parallel lines that are intersected by a transversal, and again my parallel lines are identified by using the same number of arrows, then two special angles are congruent and that is alternate interior angles. So let's examine these two words, alternate means on opposite sides, interior means within or in between. So here we have our two parallel lines, our alternate interior angles are going to be the angles that are inside and on opposite sides of the transversal. So angle 4 is inside and its opposite side would be 6 so those two angles will be congruent. There's only one other pair of alternate interior angles and that's angle 3 and its opposite side in between the parallel lines which is 5.
So alternate interior angles will always be congruent and always be on opposite sides of this transversal.