(1) (-1)+2+(-3)+4+···+(-99)+100
=(-1+2)+(-3+4)+···+(-99+100)
=(-1)*100/2=-50 --------两个一组和为 -1
(2) 3+(-6)+6+(-9)+9+···+27+(-30)
=3+(-6+6)+(-9+9)+···+(-27+27)+(-30)
=3-30= -27
(3) 2008/4=502
1+(-2)+(-3)+4+5+(-6)+(-7)+8+···+2004+2005+(-2006)+(-2007)+2008
=(1-2-3+4)+(5-6-7+8)+ ···+(2005-2006-2007+2008)
=0+0+···+0 = 0 --------四个一组和为0