As shown in the figure. When making parallel lines, the two angles are congruent and equal; There are two angles, the inner angle and the equal angle. Then use the "bisector", and the two halves are equal. Then, an isosceles triangle appeared. Write the proportion:
BA/AE? =? Is BD/DC BA/AC? =? BD/DC. This is the famous property theorem of the bisector of the triangle interior angle.