a, 1 - 2 - 3 + 4 + 5 + 6 - 7 + ... + 1996 + 1997 - 1998 - 1999 + 2000 + 2001
= ( 1 - 2 - 3 + 4 ) + ( 5 - 6 - 7 + 8 ) + ... +( 1993 - 1994 - 1995 + 1996 ) + ( 1997 - 1998 - 1999 + 2000 ) + 2001
= 0 + 0 + ... + 0 + 0 + 2001
= 2001
b, 1 - 3 + 5 -7 + ...+ 2001 - 2003 + 2005
= ( 1 -3 ) + ( 5 - 7 ) + ... + ( 2001 - 2003 ) + 2005
= -2 + (-2 )+ ... + (-2) + 2005
Có 501 số ( - 2 )
= ( - 2 ) . 501 + 2005
= -1002 + 2005
= 1003
P/s : Tham khảo ( Chép đầu bài thôi cũng sai )
a, 1 - 2 - 3 + 4 + 5 + 6 - 7 + ... + 1996 + 1997 - 1998 - 1999 + 2000 + 2001
= ( 1 - 2 - 3 + 4 ) + ( 5 - 6 - 7 + 8 ) + ... +( 1993 - 1994 - 1995 + 1996 ) + ( 1997 - 1998 - 1999 + 2000 ) + 2001
= 0 + 0 + ... + 0 + 0 + 2001
= 2001
b, 1 - 3 + 5 -7 + ...+ 2001 - 2003 + 2005
= ( 1 -3 ) + ( 5 - 7 ) + ... + ( 2001 - 2003 ) + 2005
= -2 + (-2 )+ ... + (-2) + 2005
= ( - 2 ) . 501 + 2005
= -1002 + 2005
= 1003