1²-2²+3²-4²+...+2003²-2004²+2005²-2006²+2007²
=1²+(3²-2²)+(5²-4²)+…+(2007²-2006²)
=1+(3+2)(3-2)+(5-4)(5+4)+…+(2007-2006)(2007+2006)
=1+2+3+4+5+…+2007
=2007×2008/2
=2015028