1/(x-1)(x-2)+1/(x-2)(x-3)+.+1/(x-2004)(x-2005)
=1/(x-2)-1/(x-1)+1/(x-3)-1/(x-2)+.+1/(x-2005)-1/(x-2004)
=1/(x-2005)-1/(x-1)
=2004/[(x-2005)(x-1)]