1、 2001+2000-1999-1998+1997+1996-1995-1994+……+5+4-3-2+1
=(2001-1999)+(2000-1998)+(1997-1995)+(1996-1994)+.+(5-3)+(4-2)+1
=2*1000+1
=2001
3.5 27 / 326-2 27 / 326+1 1819*（78-87.5%)
=(5 27-2 27 )/326+1 1819*0
=26
4.31*2930+61 111 / 12
=(30+1)*2930+(60+12/11) / 12
=29+29/30+5+1/11
=35+19/330