999+99+9
=(1000-1)+(100-1)+(10-1)
=1000+100+10-(1+1+1)
=1100+10-(2+1)
=1110-3
=1107
456-99-98
=456-(100-1)-(100-2)
=456-100-100+1+2
=356-100+1+2
=256+1+2
=257+2
=259
111+102+103
=(100+11)+(100+2)+(100+3)
=(100+100+100)+(11+2+3)
=(200+100)+(13+3)
=300+16
=316
721-398
=721-(400-2)
=721-400+2
=321+2
=323
691+842+309
=691+309+842
=1000+842
=1842
476+94-76
=476-76+94
=400+94
=494
757-(234+457)
=757-234-457
=757-457-234
=300-234
=66
672-278-22
=672-(278+22)
=672-300
=642
或是
=672-22-278
=650-278
=642
1+2+3+4+5+6+7+8+9+10
=(1+9)+(2+8)+(3+7)+(4+6)+(5+10)
=10+10+10+10+15
=4*10+15
=40+15
=55
29+2+28+22+27+21
=(29+21)+(28+22)+(27+2)
=50+50+29
=100+29
=129
1+2+3+4+5+6+1+2+3+4+5+6+1+2+3+4+5
=1*3+2*3+3*3+4*3+5*3+6*2
=3+6+9+12+15+12
=9+9+12+15+12
=18+12+15+12
=30+15+12
=45+12
=57
有不懂的地方再问我吧(*^__^*)
望采纳
追问多交流。谢谢您。以后不懂得就问您。真好楼主QQ多少
追答1500284987