A circle 9 cm in radius was folded so that semicircle might overlap exactly as a diameter AOB to be a folding line as shown in Fig. 1.
Next, semicircle was folded again so that the point C of dividing a semicircle circumference into two equally might overlap with the center O of the circle as shown in Fig. 2.
The folding line is the line segment DE. Fig.3 shows that the folded figure is spread out to be original circle.
Find the area difference of the shadow area and gray area of Fig. 3.
Pi is assumed to be 3.14.