Solution: Let the radii of the big circle and the small circle be r and r respectively, the area of the circle be s, and the area of the shadow part be s, then according to the meaning of the question:
s=pi*(r^2)-pi*(r^2),s=(r^2)/2-(r^2)/2
Then s = 2 * pi * s = 2 * 3.14 * 25 =157 (square centimeter).
(Note: PI means pi, so you can't type. Please forgive me. )