## If two parallel lines are cut by a transversal, then the two pairs of same-side interior angles are supplementary.

### When two parallel lines are intersected by a third line called a transversal, several pairs of angles are formed

When two parallel lines are intersected by a third line called a transversal, several pairs of angles are formed. In this case, we are specifically looking at same-side interior angles.

Same-side interior angles are the pairs of angles that lie on the same side of the transversal and inside the two parallel lines.

According to the Parallel Lines Transversal Theorem, if two parallel lines are cut by a transversal:

1. Corresponding angles are congruent: The pairs of angles that are on the same side of the transversal and at the same positions relative to the parallel lines are congruent.

2. Alternate interior angles are congruent: The pairs of angles that are on opposite sides of the transversal, inside the two parallel lines, and at the same positions relative to the transversal are congruent.

3. Alternate exterior angles are congruent: The pairs of angles that are on opposite sides of the transversal, outside the two parallel lines, and at the same positions relative to the transversal are congruent.

4. Same-side interior angles are supplementary: The pairs of angles that are on the same side of the transversal and inside the two parallel lines add up to 180 degrees. In other words, they are supplementary angles.

The concept of supplementary angles means that when two angles are supplementary, the sum of their measures is equal to 180 degrees.

This theorem has numerous applications in geometry and is useful to determine angle measurements and relationships when working with parallel lines and transversals.

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