第一类:加法 65+73+135 =(65+135)+73 =200+73 =273 357+288+143 =(357+143)+288 =500+288 =788 272+68+28 =(272+28)+68 =300+68 =368 129+235+171+165 =(129+171)+(235+165) =300+400 =700 17+145+23+35 =(17+23)+(145+35) =40+180 =220 999+99+9+3 =(999+1)+(99+1)+(9+1) =1000+100+10 =1110 9998+3+99+998+3+9 =(9998+2)+(99+1)+(998+2)+(9+1) =10000+100+1000+10 =11110 第二类:减法 400-256-44 =400-(256+44) =400-300 =100 517-53-47 =517-(53+47) =517-100 =417 284-159-41 =284-(159+41) =284-200 =84