算式1+2+3+4+5+6+7+8+9+8+7+6+5+4+3+2+1分之999999999*999999999
=(999999999*999999999)/(1+2+3+4+5+6+7+8+9+8+7+6+5+4+3+2+1)
=(111111111*9*111111111*9)/81
=111111111*111111111/1
=12345678987654321/1
=12345678987654321