1/x^2-3x+2-1/x^2-5x+6-1/x^2-7x+12-1/x^2-9x+20

1/x^2-3x+2-1/x^2-5x+6-1/x^2-7x+12-1/x^2-9x+20
=1/(x-1)(x-2)-1/(x-2)(x-3)-1/(x-3)(x-4)-1/(x-4)(x-5)
=1/(x-1)-1/(x-2)-[1/(x-2)-1/(x-3)+1/(x-3)-1/(x-4)+1/(x-4)-1/(x-5)]
=1/(x-1)-1/(x-2)-1/(x-2)+1/(x-5)
=1/(x-1)-2/(x-2)+1/(x-5)
(此题中间的三个减号是否有错?）
再问： 没有啊，都是减号