B2: a) S1=(1-2)+(3-4)+......+(2001-2002)
=(-1)+(-1)+......+(-1) . Co 1001 so hang
=-1 .1001
=-1001
b) S2=(1-3)+(5-7)+.......+(1997-1999)
=(-2)+(-2)+.......+(-2) . Co 500 so hang
=-2.500
=-1000
c) S3=(1-2-3+4)+.....+(1997-1998-1999+2000)
=0+......+0
=0
B3: a)A=a+b-a+b+a-c-a-c
A=[(a-a)+(a-a)]+[(b+b)+(-c-c)]
A=2b -2c
A=4.(b-c)
b)B=a+b-c+a-b+c-b-c+a-a+b+c
B=[(a-a)+(a+a)]+[(b-b)+(-b+b)]+[(-c+c)+(-c+c)]
B=0+2a+0+0+0+0
B=2a
