第一题是化简吧,如下：1/（x-1）+ 1/(x-1)(x-2) + 1/(x-2)(x-3) + 1/(x-3)(x-4)= 1/(x-1) * [1 + 1/（x-2)] + 1/(x-2)(x-3) + 1/(x-3)(x-4)= 1/(x-1) * [(x-1)/（x-2)] + 1/(x-2)(x-3) + 1/(x-3)(x-4)= 1/(x-2) + ...