设 $x, y$ 是正数, 且 $x^{y}=y^{x}, y=9 x$, 则 $x$ 的值为 ( )
案】选 B $\because x^{9 x}=(9 x)^{x}, \quad\left(x^{9}\right)^{x}=(9 x)^{x}$,
$\therefore x^{9}=9 x . \therefore x^{8}=9 . \therefore x=\sqrt[8]{9}=\sqrt[4]{3}$.