9. As shown in the figure, the diameter $A B$ of circle $O$ is perpendicular to $C D$, $A B=10$ cm, and an arc is drawn with $C$ as the center and $C A$ as the radius. Find the area of the crescent shape $A D B E A$ (the shaded part).

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9 Solution: (1) The area of the crescent $\mathrm{ADBEA}$ (shaded part) = the area of the semicircle + the area of $\triangle \mathrm{ABC}$ - the area of sector $C \overline{A E B C}$
(2) The area of the crescent ADBEA $=\frac{1}{2} \times \pi \times 5^{2}+25-\frac{1}{4} \times \pi \times 50=25$ (square centimeters).

Answer: The area of the crescent ADBEA is 25 square centimeters.