999 x 998998999 - 998 x 999999998

=999 x (998998998+1) - 998 x (999999999 -1)
=（999 x 998 x 1001001 +999）-（998 x 999 x1001001 -998)
=998 + 999
=1997

解:原式=999*(998*1001001+1)-998*(999*1001001-1)
=999*998*1001001+999-998*999*1001001+998
=999+998
=1997

999 x 998998999 - 998 x 999999998
=999 x （999999999-1001000） - （999-1） x （999999999-1 ）
=999 x 999999999-999999000-999 x 999999999+999+999999999-1
=2x999-1
=2000-3
=1997

(998+1)*998998999-998*999999998=998*(-999999998+998998999)+998998999=-1000999*998+998998999=-998997002+998998999=1997

999 x 998998999 - 998 x 999999998
=999x(998000000+998000+999)-998x(999000000+999000+998)
=999x998000000+999x998000+999x999-998x999000000-998x999000-998x998

999x998000000=999x998x1000000=998x999000000
999x998000=999x998x1000=998x999000

所以原式=999x999-998x998=(999+998)x(999-998)=1997

不知道说得是否明白

999 x 998998999 - 998 x 999999998
=999 x (998 x 1001001+1