1.9+19.9+199.9+1999.9+0.4=2-0.1+20-0.1+200-0.1+2000-0.1+0.4=(2+20+200+2000)-(0.1×4-0.4)=2222
把0.4拆成四个0.1＝2+20+200+2000＝2222
=(2-O.1)十(20-O.1)十(200一O.1)十(2000-O.1)十0.4 =2十20十200十2000 =2222
