已知:△ABC是边长为4的等边三角形,点O在边AB上, 圆O过点B且分别与边AB,BC相交于点D,E,EF⊥AC垂足为F,(1)求证:直线EF是圆O的切线;(2)当直线DF与圆O相切时,求圆O的半径
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