8. Given the function $f(x)=\log _{\frac{1}{2}} x$, let $x=\frac{a}{f(a)}$, $y=\frac{b}{f(b)}, z=\frac{c}{f(c)}(0<c<b<a<1)$. Then, the order of $x 、 y 、 z$ in terms of size is $\qquad$ .
$$
\begin{array}{l}
\text { 8. } x>y>z . \\
x-y=\frac{a \log \frac{1}{2} b-b \log _{\frac{1}{2}} a}{\log \frac{1}{2} a \cdot \log \frac{1}{2} b} \\
=\frac{(a \lg b-b \lg a)(-\lg 2)}{\lg a \cdot \lg b}=\frac{-\lg 2 \cdot \lg \frac{b^{a}}{a^{b}}}{\lg a \cdot \lg b} . \\
\text { From } 0<y$.
Similarly, $y>z$.
$$