Let DBC be angle 1, DCB be angle 2, ABE= EBD be angle 3, and ACE= ECD be angle 4. Three relationships can be listed through three triangles: angle 1+ angle 2+ angle D= 180, angle 1+ angle 3+ angle 2. Angle 1+2, angle 3+ angle 2+2, angle 4+ angle A= 180. Through the right side of these three formulas, we find that the second formula is multiplied by 2(2 times 180)= the sum of the first two formulas (180+ 180).