When two parallel lines are cut by a transversal, what is true about alternate exterior angles?

When two parallel lines are cut by a transversal, what is true about alternate exterior angles?

• A. They are congruent.
• B. They are supplementary.
• C. They are perpendicular.
• D. They are vertical angles.

Answer: (A)

When a transversal crosses two parallel lines, the angle the transversal makes with each line in a given relative position stays the same. Alternate exterior angles are those outside the two lines on opposite sides of the transversal, so they mirror each other in the same way. This preservation of orientation forces them to have equal measures, so they are congruent. They’re not vertical (that term applies to angles at the same intersection), not necessarily perpendicular, and not supplementary in this situation.

When a transversal crosses two parallel lines, the angle the transversal makes with each line in a given relative position stays the same. Alternate exterior angles are those outside the two lines on opposite sides of the transversal, so they mirror each other in the same way. This preservation of orientation forces them to have equal measures, so they are congruent. They’re not vertical (that term applies to angles at the same intersection), not necessarily perpendicular, and not supplementary in this situation.