Nine, find the minimum value of $f(x)=\frac{9 x^{2} s \sin ^{2} x+4}{x \sin x}(0<x<\pi)$.
Let $\mathbf{x} \sin \mathrm{x}=\mathrm{t}$, then $\mathrm{t}$ is a real number.
Thus $\quad \mathrm{f}(\mathrm{x})=\frac{9 \mathrm{t}^{2}+4}{\mathrm{t}} \geq \frac{2 \cdot 3 \cdot 2}{t}=12$.
$\therefore \mathrm{f}(\mathrm{x})$'s minimum value is 12.