A=1-3+5-7+.......+2001-2003+2005
A = (1 - 3) + (5- 7) +.... + (2001 - 2003) + 2005
A = -1 + -1 +....+ -1 + 2005 ( 503 số hạng -1)
A = -1.503 + 2005 = -503 + 2005 = 1502
B=1-2-3+4+5-6-7+8......+1993-1994
B = (1-2-3+4) + (5-6-7+8) ......... + (1990 -1991 - 1992 + 1993) -1994
B =0 + 0+0+0 -1994 = -1994
C = 1+2-3-4+5+6-7-8+9+.....+2002-2003-2004+2005+2006
C = 1 + (2-3-4+5) +(6-7-8+9)+......+(2002-2003-2004+2005) +2006
C = 1 + 0+0+0+........+ 2006 = 1+2006 =2007
S1 =1+(-2)+3+(-4)+......+(-2014)+2015
S1 =[1+(-2)]+[3+(-4)]+......+[2003+(-2014)]+2015
S1 = -1 + -1 + -1 + ........ + 2015 (1007 số hạng -1)
S1 = -1.1007 + 2015 = -1007 + 2015 = 1008
