8. Let $a_{n}$ be the coefficient of the $x$ term in the expansion of $(3-\sqrt{x})^{n}$ $(n=2,3,4, \cdots)$, then $\lim _{n \rightarrow \infty}\left(\frac{3^{2}}{a_{2}}+\frac{3^{3}}{a_{3}}+\cdots+\frac{3^{n}}{a_{n}}\right)$ $=$ . $\qquad$
18
8.【Analysis and Solution】From the problem, by the binomial theorem, we have $a_{n}=\mathrm{C}_{n}^{2} 3^{n-2}, \therefore \frac{3^{n}}{a_{n}}=\frac{3^{2} \times 2}{n(n-1)}=18\left(\frac{1}{n-1}-\frac{1}{n}\right)$, $\therefore \lim _{n \rightarrow x}\left(\frac{3^{2}}{a_{2}}+\frac{3^{3}}{a_{3}}+\cdots+\frac{3^{n}}{a_{n}}\right)=\lim _{n \rightarrow x} 18\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\cdots+\frac{1}{n-1}-\frac{1}{n}\right)=\lim _{n \rightarrow x} 18\left(1-\frac{1}{n}\right)=18$.