11. As shown in the figure, the diameter $O A$ of circle $P$ is the radius of circle $O$, $O A \perp B C, O A=10$, then the area of the shaded part is $\qquad$ .$(\pi$ is taken as 3)
【Solution】Solve: $3 \times 10^{2} \div 2-3 \times(10 \div 2)^{2}$
$$
\begin{array}{l}
=3 \times 100 \div 2-3 \times 25 \\
=150-75 \\
=75
\end{array}
$$

Answer: The area of the shaded part is 75.
Therefore, the answer is: 75.