9. As shown in the figure, 4 identical right-angled triangles are pieced together to form a windmill shape. It is known that $E, F, G, H$ are the midpoints of $OA, OB, OC, OD$ respectively, and $AC=BD=20$ cm. What is the area of this windmill in $\qquad$ square centimeters?
【Analysis】As shown in the figure, connecting $\mathrm{AD}$, $\mathrm{DC}$, $\mathrm{CB}$, and $\mathrm{BA}$, we can see that $\mathrm{ABCD}$ is a square, and the area of the square is $20 \times 20 \div 2=200$. Since $\mathrm{E}$ is the midpoint of $\mathrm{OA}$, we have $\mathrm{AE}=\mathrm{EO}$, so $S_{\triangle A E D}=S_{\triangle O E D}$. Similarly, $S_{\triangle B F A}=S_{\triangle O F A}$, $S_{\triangle C G B}=S_{\triangle O G B}$, and $S_{\triangle D H C}=S_{\triangle O H C}$. Therefore, the area of the windmill shape is half the area of the square $\mathrm{ABCD}$, which is 100 square centimeters.