原式=999...9(1998个9)*999...9(1998个9)+1999...9(1998个9)
=999...9(1998个9)*（1000...0(1998个0)-1）+1000...0(1998个0)+999...9(1998个9)
=999...9000..0(1998个9接1998个0)-999...9(1998个9)+1000...0(1998个0)+999...9(1998个9)
=999...9000..0(1998个9接1998个0)+1000...0(1998个0)
=1000...0(1998x2个0)
=1000...0(3996个0)