\[\frac{1}{(x - y)(y - z)} - \frac{1}{(y - z)(x - z)} - \frac{1}{(z - x)(y - x)} = 0\]
\[\frac{1^{\backslash x - z}}{(x - y)(y - z)} - \frac{1^{\backslash x - y}}{(y - z)(x - z)} - \frac{1^{\backslash y - z}}{(x - z)(x - y)} = 0\]
\[\frac{x - z - x + y - y + z}{(x - y)(y - z)(x - z)} = 0\ \ \ \]
\[0 = 0\]
\[ч.т.д.\]