1+2+3+4+...... +2004+2005+2004+....+4+3+2+1
=1+2+3+4+...... +2004+2005+2005+2004+....+4+3+2+1-2005
=2*(1+2+..+2005)-2005
=2*(1+2005)*2005/2-2005
= 4020025
1+2+3+4+...... +2004+2005+2004+....+4+3+2+1
=(1+2004)+(2+2003)+..+(2004+1)+2005
=2005x2005
=4020025
(1+2004)*2004+2005=2005*2005=4020025
=2(1+2+3+…+2004)+2005
=2(1+2004)1002+2005
=2005(1002×2+1)
=4020025