平行线分线段成比例!

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平行线分线段成比例!
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平行线分线段成比例!
平行线分线段成比例!
 

平行线分线段成比例!
AB:EF=DB:DF
(AB-EF):EF=(DB-DF):DF
(AB-EF):EF=BF:DF
EF:(AB-EF)=DF:BF
(EF+AB-EF):(AB-EF)=(DF+BF):BF
AB:(AB-EF)=BD:BF=CD:EF
AB:(AB-EF)=CD:EF
a:(a-c)=b:c
ac=ab-bc
ab=bc+ac,
两边同除以ab:
1=c/a+c/b,
两边同除以c:
1/c=1/a+1/b