First, AE is perpendicular to the reverse extension line of CD at E, and similarly, AF is perpendicular to the reverse extension line of BD at F..

In the second step, because angle ADB= angle ADC, angle ADF= angle ADE, and because angle AED and angle AFD are right angles, and AD is the common side, right-angled triangle AED is equal to right-angled triangle AFD, and a conclusion is drawn: AE=AF.

In the third step, because AB = AC and AE = AF, all right-angled triangles ABF are equal to right-angled triangles ACE, and the angle ABF= angle ACE is obtained.

Because angle ABC= angle ACB, angle ABF= angle ACE, angle DBC= angle DCB.