As shown in the figure, four points a, F, C and D are on a straight line, AF = CD, ab ‖ De, and ab = De

As shown in the figure, four points a, F, C and D are on a straight line, AF = CD, ab ‖ De, and ab = De

It is proved that: (1) AF = CD, AF + FC = CD + FC is AC = DF.