999...9（1999个9）*999...9（1999个9）+1999...9（1999个9)
=999...9^2(1999个9)+1999...98(1999-1个9)+1
=999...9^2(1999个9)+2*999...9(1999个9)*1+1^2
=(999...9(1999个9)+1)^2
=1000...0^2(1999个0)
=1000...0(2*1999个0)