As shown in the figure, ad ∥ BC, ∠ a = ∠ C, be and DF divide ∥ ABC and ∥ CDA equally

It is proved that: ∵ ad ∥ BC, ∵ a + ∥ ABC = 180 °, ∵ C + ∥ ADC = 180 °, ∵ a = ∥ C, ∥ ABC = ∥ ADC, ∵ be, DF divide ∥ ABC and ∥ CDA equally, ∥ EBC = 12 ∥ ABC, ∥ EDF = 12 ∥ ADC, ∥ EBC = ∥ EDF, ∥ DFC = ∥ EDF, ∥ EBC = ∥ DFC, ∥ be ∥ DF

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