1/（x-2)（x-3)-2/(x-1)(x-3)+1/(x-1)(x-2)
=1/(x-3)-1/(x-2)-[1/(x-3)-1/(x-1)]+1/(x-2)-1/(x-1)
=1/(x-3)-1/(x-2)-1/(x-3)+1/(x-1)+1/(x-2)-1/(x-1)
=0