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證明: 過D做直線DH和AC平行,交BC在H點. 那麼就有△DEH和△FEC相似.囙此就有DH/FC=DE/EF 又因為DH和AC平行,△ABC是等腰三角形,囙此就有△DBH也是等腰三角形.即DB=DH.再根據上述的結論,就可以得出DE/EF=DB/CF 證畢.
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