1+（-3）+5+（-7）+.+2001+（-2003）+2005=?我要过程

原式=(1-3)+(5-7)+(9-11)+.+(2001-2003)+2005 (其中有501个括号,即有501组)
=(-2)*501+2005
= -1002+2005
=1003

看成2个等差数列也可以.
1,5,9……2001，2005
-3，-7……-1999，-2003
然后根据公式做出来

1 + (-3) = -2 ..................................4*0 + 1
5 + (-7) = -2 ..................................4*1 + 1
9 + (-11) = -2 ..................................4*2 + 1
......
2001 + (-2003) = -2 ............................ 4*500 + 1
-2*501 = -1002
-1002 + 2005 ＝ 1003