1. Using the concept of right angle, it is proved that two adjacent angles are complementary;

2. Make a straight line through two of the three points, and prove that the third point is on this straight line;

3. (Make the straight line MN and AC intersect at b) If the angle ABM= the angle CBN (or the angle ABN= the angle CBM), then the three points A, B and C are * * * lines;

4. Apply the inverse theorem of Menelaus theorem.