ПОМОГИТЕ ДАМ 30 БАЛЛОВ

Просто приводим к одному знаменателю
$\frac{1}{(x-y)(y-z)} - \frac{1}{(y-z)(x-z)} - \frac{1}{(z-x)(y-x)} = \\ = \frac{1}{(x-y)(y-z)} - \frac{1}{(y-z)(x-z)} - \frac{1}{(x-z)(x-y)} = \\ =\frac{x-z}{(x-y)(y-z)(x-z)} - \frac{x-y}{(y-z)(x-z)(x-y)} - \frac{y-z}{(x-z)(x-y)(y-z)} = \\ =\frac{x-z-x+y-y+z}{(x-y)(y-z)(x-z)} =0$

$\frac{1}{(a-b)(a-c)} + \frac{1}{(b-a)(b-c)} + \frac{1}{(c-a)(c-b)} = \\ = \frac{1}{(a-b)(a-c)} - \frac{1}{(a-b)(b-c)} + \frac{1}{(a-c)(b-c)} =\\ = \frac{b-c}{(a-b)(a-c)(b-c)} - \frac{a-c}{(a-b)(b-c)(a-c)} + \frac{a-b}{(a-c)(b-c)(a-b)}= \\ =\frac{b-c-a+c+a-b}{(a-b)(a-c)(b-c)} =0$