I 13 years old in 2011.
①△BDF和△CEF都是等腰三角形;②DE=BD+CE;③△ADE的周长等于AB与AC的和;④BF=CF.其中正确的有____.(填正确的序号)
— It's fine today, Jim. Shall we go to the zoo?
— ______.
关于摩擦力,下列说法正确的是( )