9. In the sequence $\left\{a_{n}\right\}$, if $a_{1}=3$, and $n a_{n}=(n+1) a_{n+1}$, then $a_{n}=$
9. $a_{n}=\frac{3}{n}$. Let $b_{n}=n a_{n}$, therefore $b_{n+1}=b_{n}$, so $b_{n}=b_{1}=a_{1}=3$. Hence $a_{n}=\frac{3}{n}$.