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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