|4a+5|-|a-4|当a<-5/4时：|4a+5|-|a-4|=-4a-5-4+a =-3a-9当-5/4 ≤ a ≤ 4时：|4a+5|-|a-4|=4a+5-4+a =5a+1当4 =3a+9
丨4a+5 丨-丨a-4丨 么
