I3.4 Let $f(0)=0 ; f(n)=f(n-1)+3$ when $n=1,2,3,4, \ldots$. If $2 f(S)=R$, find the value of $S$.
$\begin{array}{l}f(1)=0+3=3, f(2)=3+3=3 \times 2, f(3)=3 \times 3, \ldots, f(n)=3 n \\ R=3996=2 f(S)=2 \times 3 S \\ S=666\end{array}$