If two coplanar lines are cut by a transversal and the lines are parallel, the same-side interior angles are

If two coplanar lines are cut by a transversal and the lines are parallel, the same-side interior angles are

• A. Supplementary
• B. Congruent
• C. Complementary
• D. Vertical

Answer: (A)

Same-side interior angles are the interior angles on the same side of the transversal between two parallel lines. When lines are parallel, those two interior angles lie along a straight line through the transversal, so their measures add to 180 degrees. They are therefore supplementary. The other options don’t fit because these angles aren’t necessarily equal (that would be true for alternate interior or corresponding angles), they don’t have to sum to 90, and vertical angles are the opposite angles at a single intersection, not the two interior angles on the same side between the lines.

Same-side interior angles are the interior angles on the same side of the transversal between two parallel lines. When lines are parallel, those two interior angles lie along a straight line through the transversal, so their measures add to 180 degrees. They are therefore supplementary. The other options don’t fit because these angles aren’t necessarily equal (that would be true for alternate interior or corresponding angles), they don’t have to sum to 90, and vertical angles are the opposite angles at a single intersection, not the two interior angles on the same side between the lines.