2000+2001分之1加2001+2002分之1+2002+2003分之1+2003分之1
=1/2000×2001+1/2001×2002+1/2002×2003+1/2003
=1/2000-1/2001+1/2001-1/2002+1/2002-1/2003+1/2003
=1/2000