Question

1+2-3+4+5-6+7+8-9+...+2005

Answer 1

(1+2-3)+(4+5-6)+(7+8-9)+.+(2002+2003-2004)+2005

=0+3+6+.+2001+2005

=等差数列a1=3,公差3,项数667求和+2005

=(3+2001)*667/2+2005

=668334+2005

=670339

有助可采.