999*2224+6666*1664,
=(1000-1)*2224+6666*(1000+664)
=1000*2224-2224+6666*1000+6666*664
=(2224+6666)*1000+6666*664-2224
=8890*1000+4426224-2224
=8890000+4424000
=13314000

第一题没什么好办法,最好硬算

2012*2012-2010*2011,
=(2010+2)*2012-2010*2011
=2010*2012+2*2012-2010*2011
=2010*(2012-2011)+2*2012
=2010+4024
=6034



1999+199.9+19.99+1.999
=(2000-1)+(200-0.1)+(20-0.01)+(2-0.001)
=2000+200+20+2-(1+0.1+0.01+0.001)
=2222-1.111
=2220.889