998+997+996+1002+1003+1004
=(998+1002)+(997+1003)+(996+1004)
=2000+2000+2000
=6000
998+997+996+1002+1003+1004
=998+1002+997+1003+996+1004
=(998+1002)+(997+1003)+(996+1004)
=2000+2000+2000
=6000
(998+2)-2+(997+3)-3+(996+4)-4+1000+2+1000+3+1000+4
=6000
从左到右顺,3499899979996999000加10000加一零