A. The side of a triangle that forms a side of two given angles.

B. A closed figure formed by three or more coplanar segments, sides with common endpoints that are noncollinear, and each side intersects exactly two other sides, but only at their endpoints.

C. A proof which organizes a series of statements in logical order, starting with the given statements. Each statement along with its reasons is written in a box.

D. 1. In a right triangle, the sides opposite the acute angles. 2. In an isosceles triangle, the congruent sides. 3. In a trapezoid, the nonparallel sides.

E. 1. For an angle, the two rays that form the sides of the angle. 2. For a polygon, a segment joining two consecutive vertices of the polygon.