A. A line or line segment added to a given figure to help in proving a result.

B. 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.

C. A triangle all of whose angles are acute angles.

D. The angles of a triangle that are not adjacent to a given exterior angle.

E. In a triangle, the angle formed by two sides is the included angle for these two sides.