The proof process is as follows:

1, branch 1: angle A= angle ACD- angle b, which can be seen from the dividing line of two angles: angle A 1BC= angle B/2, angle A 1CD= angle ACD/2;

2. So: Angle A 1= Angle A 1CD- Angle A 1BC= Angle ACD/2- Angle B/2= (Angle ACD- Angle B)/2 = Angle A * (1/2);

Namely: angle A 1 = angle A * (1/2)-1.

Similarly, we can get: angle A2= angle A * (1/2) (1/2)-2 (1/2).

Angle A3= angle a * (1/2) (1/2) (1/2)-3 (1/2).

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Angle An= angle a * (1/2) (1/2) (1/2) .................... n (1/2).