2004+2003-2002-2001+2000+1999-1998-1997+...+8+7-6-5+4+3-2-1 And 2005x2006-2006x20052005 How to calculate easily?

2004+2003-2002-2001+2000+1999-1998-1997+...+8+7-6-5+4+3-2-1 And 2005x2006-2006x20052005 How to calculate easily?

1. Every 4 groups,2004+2003-2002-2001+2000+1999-1998-1997+...+8+7-6-5+4+3-2-1=(2004+2003-2002-2001)+(2000+1999-1998-1997)+...+(4+3-2-1)=4+4+...+4=4×(2004÷4)=2004 2. The title is wrong. The first one is 2005×2006? 2005×2...

1+2-3-4+5+6-7-8+9+10…… -1999-2000+2001+2002-2003-2004+2005+2006[ Simple calculation] Is [] equal everywhere between parallel lines? The vertical segment is the shortest segment from a point outside the line to the line. This vertical segment is called point to line []? Perimeter formula of diamond? Area formula of sector? [To normal formula! Don't get me any messy symbols! ]

(1+2-3-4)(5+6-7-8)... Every four numbers are a group, the number is -4, there are (2006-2)/4=501 groups, so the formula is:1+2-3-4+5+6-7-8+9+10-11-12+.-1999-2000+2001+2003-2004+2005+2006=(1+2-3-4)+(5+6-7-8)+... +(2001+2002-2003...