(1)原式=(1-2-3+4)+(5-6-7+8)+(9-10-11+12).+(2005-2006-2007+2008)
=0
(2)原式=-(1/3-1/2)-(1/4-1/3)-(1/5-1/4)-(1/6-1/5)-.-(1/2008-1/2007)
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+.+1/2006-1/2006+1/2007-1/2007-1/2008
=1/2-1/2008
=1004/2008-1/2008
=1003/2008
(3)原式=(1-1/2)+(1/2-1/3)+(1/3-1/4)+.+(1/2007-1/2008)
=1-1/2+1/2-1/3+1/3-1/4+.+1/2007-1/2008
=1-1/2008
=2007/2008
(4)1/n-1/n+1=1/n(n+1)-----等号右边的n+1作为分母