SOLUTION
For
4. We see that angles A, D and ABD are angles in a triangle.
So also angles C, E, and CBE.
Therefore, the reason here is the sum of angles in a triangle.
5. Now, angle ABD, angle DBE, and angle CBE all sum up to 180 degrees, and they all lie on a straight line.
Therefore, the reason here is; angles on a straight line.