题目

如图,D是等边△ABC的边BC上一动点,ED∥AC交AB于点E.DF⊥AC交AC于点F,DF=,若△DCF与E、F、D三点组成的三角形相似,则BD的长等于_____. 答案:【答案】3或1【解析】由于△DCF与E、F、D三点组成的三角形相似,则需要分情况讨论,因为ED//AC,所以,①当时,DF=,,则而易得是等边三角形,从而;②当时,同理求出Scientists say it may be five years ______ it is possible to test this medicine on human patients.     A. since                B. until                C. before               D. when
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