999.9*999.9+199.99巧算

999.9*999.9+199.99
=(1000-0.1)*999.9+199.99
=999900-99.99+199.99
=999900+100
=1000000
（10.1-1.009-2）*0.49+0.51*（10.1-1.009+1）
=10.1*0.49-1.009*0.49-2*0.49+0.51*10.1-0.51*1.009+0.51*1
=10.1*(0.49+0.51)-1.009*(0.49*0.51)-2*(0.49+0.51)+0.51*3
=10.1-1.009-2+1.51
=10.1-1.009-0.49
=10.1-1.499
=10.1-1.5+0.001
=8.6+0.001
=8.601
111.1(2002个）*999...9（2002个）
=111.1(2002个）*1000000(2002个0）-111.1(2002个）*1
=111.1(2002个1）00000...00（2002个）-111.1(2002个）
=111...1（2001个1）888888（2001个8）9