9. As shown in the figure, $A B C D$ and $E G F O$ are both squares, where point $O$ is the center of square $A B C D$, and $E F / / B C$. If the lengths of $B C$ and $E F$ are both positive integers, and the area of quadrilateral $B C E F$ is 3.25, then $\mathrm{S}_{A B C D}-\mathrm{S}_{E G F O}=$ $\qquad$ ($\mathrm{S}_{E G F O}$ represents the area of $E G F O$, and so on).
【Analysis】Let $B C=a, E F=b$, then we have
$$
\begin{array}{l}
S_{\text {shaded }}=\frac{b^{2}}{4}-\frac{a^{2}}{4}=3.25 \\
b^{2}-a^{2}=13 \\
\therefore(b+a)(b-a)=13 \\
\left\{\begin{array} { l } 
{ b - a = 1 } \\
{ b + a = 13 }
\end{array} \Rightarrow \left\{\begin{array}{l}
b=7 \\
a=6
\end{array}\right.\right. \\
S_{A B C D}-S_{E G F O}=36-7 \times 7 \div 2=11.5 \\
\end{array}
$$