Because AE is an angular bisector, Be is perpendicular to AD and E..

Triangle ABF is an isosceles triangle, AB=AF,

So (ac-ab) = cf.

The sum of the internal angles of the triangle is 180:

Angle ABC++ Angle C+Angle BAC =180;

Angle ABC = 90- 1/2 Angle BAC+2× Angle FBC.

Then angle FBC= = angle C.

So BF = cf

Because be = 1/2bf.

Therefore, the result of equivalent substitution is BE=2/ 1(AC-AB).

Therefore, AC-AB=2BE is obtained by equivalent substitution.

Extend the intersection of be and AC to F, because AD is the bisector of angle BAC and BE is perpendicular to AD and E, so it is easy to get AF=AB and angle AFB= angle ABF, because angle AFB= angle FBC+ angle C, and angle FBC is added to both sides of the equation at the same time, that is, angle AFB+ angle FBC= angle ABF+ angle FBC= angle ABC = angle FBC+ angle C, because