2. The shortest line segment between two points.

3. On the same plane, there is one and only one straight line perpendicular to the known straight line.

4. The same angle is equal, and two straight lines are parallel.

5. There is one and only one straight line parallel to this straight line.

6. A triangle with two sides and an equal included angle is congruent.

7. A triangle with two equal angles and congruent sides.

8. A triangle with two equal sides is congruent.