Rules for Line Intersections (Jump to: Lecture | Video )

Figure 1.

Above, we have a line that is intersecting two parellel lines. Numbers in purple represent Interior Angles, while numbers in red represent Exterior Angles.

There are three rules to remember when a line intersects two parallel lines:

1. Alternate interior angles are congruent.

In this case, the alternate interior angles are 3 / 6, and 4 / 5. To say that they are congruent is to say that they are equal.

2. Alternate exterior angles are congruent.

In this case, the alternate exterior angles are 1 / 8, and 2 / 7. To say that they are congruent is to say that they are equal.

3. Corresponding angles are congruent.

In this case, the corresponding angles are 1 / 5, 2 / 6, 3 / 7, and 4 / 8. To say that they are congruent is to say that they are equal.