Please adopt me, I don't need to use equations at all. Besides, your pictures are not good at all. The radius of the circle should be OA, you didn't draw it, so there is no solution at all, or you missed the condition.

A:

According to the meaning of the question, OA is long as

10X2/(8-3)=4 (cm)

The shaded area is:

1/3X4Xπ^2= 12π

So the shadow area is 12π.