Notes: Include multi-step proofs and algebraic problems built upon these concepts. Examples of theorems include but are not limited to: Angle Relationships: The sum of the interior angles of a triangle is 180 degrees. The measure of an exterior angle of a triangle is equal to the sum of the two non-adjacent interior angles of the triangle. Side Relationships: The length of one side of a triangle is less than the sum of the lengths of the other two sides. In a triangle, the segment joining the midpoints of any two sides will be parallel to the third side and half its length. Isosceles Triangles: Base angles of an isosceles triangle are congruent.