题目内容
如图,在△ABC中,BD、CD分别平分∠ABC、∠ACB,过点D作直线平行于BC,分别交AB、AC于点E、F,当∠A的位置及大小变化时,线段EF和
的大小关系是( )

A.
B.
C.
D.不能确定
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
A.


C.

B
由BD平分∠ABC得,∠EBD=∠DBC.∵ EF∥BC,∴ ∠EDB=∠DBC,
∴ ∠EBD=∠EDB,∴ △BED是等腰三角形,∴ ED=BE.同理可得,DF=FC,
∴ EF=ED+DF=BE+FC,故选B.
∴ ∠EBD=∠EDB,∴ △BED是等腰三角形,∴ ED=BE.同理可得,DF=FC,
∴ EF=ED+DF=BE+FC,故选B.

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