As shown in the figure, in triangle ABC, De is parallel to BC, EF is parallel to CD

As shown in the figure, in triangle ABC, De is parallel to BC, EF is parallel to CD

prove:
∵DE∥BC
∴△ADE∽△ABC
∴AD/AB=AE/AC
∵EF∥CD
∴△AFE∽△ADC
∴AF/AD=AE/AC
∴AF/AD=AD/AB
∴AF:AD=AD:AB
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