When a transversal crosses parallel lines, special angle pairs are formed — alternate interior, corresponding, and co-interior angles.
Lines in the same plane that never intersect are parallel. Notation: l ∥ m. The distance between them is constant everywhere.
A transversal is a line that crosses two or more lines. It creates 8 angles when it crosses two lines.
Corresponding angles are equal (same position at each intersection). Alternate interior angles are equal (Z-shape). Co-interior (same-side interior) angles are supplementary (sum to 180°).
If corresponding angles are equal, or alternate interior angles are equal, or co-interior angles sum to 180°, then the lines are parallel.
Drag to change transversal angle — watch the angle pairs stay equal
Q1: A transversal crosses two parallel lines. One angle is 70°. Its alternate interior angle is:
Q2: Co-interior angles formed by a transversal and parallel lines:
Q3: If corresponding angles are NOT equal, the lines are: