A polygonal region is the union of a polygon and its interior. A convex polygon is one that is convex as a region. A concave polygon is a non-convex polygon. Triangles are classified according to the measures of their angles as acute, right or obtuse, and according to the lengths of their sides as scalene, isosceles, or equilateral. A circle is a set of points in a plane each of which lies at the same distance from a given point, called the center.
Theorems involving angles A. Supplements of the same angle, or of congruent angles, are congruent.
B. Complements of the same angle, or of congruent angles, are congruent. C. Vertical angles formed by intersecting lines are congruent. Use Theorem A above to prove this. D. Use theorems A, B, and C above to prove the following statements. If any two distinct lines are cut by a transversal, then a pair of corresponding angles, alternate interior angles, or alternate exterior angles are congruent if, and only if, the lines are parallel.