已知,AC = (5/9)BC ,AD = (9/5)DB ,可得：AC/CB = 5/9 = DB/AD ,则有：AC/(AC+CB) = DB/(DB+AD) = 5/(5+9) ,即有：AC/AB = DB/AB = 5/14 ,所以,AC = DB = (5/14)AB ,可得：CD = AB-AC-DB = (2/7)AB ,所以,AB = (7/2)...