I1.4 Let $f(x)$ be a function such that $f(n)=(n-1) f(n-1)$ and $f(n) \neq 0$ hold for all integers $n \geq 6$. If $U=\frac{f(T)}{(T-1) f(T-3)}$, find the value of $U$.
Reference: 1999 FI5.4
$$
\begin{array}{l}
f(n)=(n-1) f(n-1)=(n-1)(n-2) f(n-2)=\ldots \ldots . \\
U=\frac{f(11)}{(11-1) f(11-3)}=\frac{10 \times 9 \times 8 \times f(8)}{10 \times f(8)}=8 \times 9=72
\end{array}
$$