The above is the original picture of the topic. I'm just talking about the property theorem of triangle bisector.

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.