The area of a small semicircle (diameter 20cm): 3.14× (20/2 )× (20/2) ÷ 2 =157cm2.
The area of overlapping shadow of two small semicircles is: 157-20×(20/2)÷2=57 cm2.
(Hint: You can divide the shadow like a leaf in the middle into two parts evenly, and then turn it over to make it in a whole circle. After careful observation, it is easy to understand why this is so. )
The shadow area of the other part is: 314-157× 2+57 = 57cm2.
(Note: When the area of a whole circle (two small semicircles) is subtracted from the area of 1/4 big circle (the whole figure), the area of the overlapping shadow part is reduced again, so it is added back to 57 square centimeters. )
Therefore, the total area of the shadow area is: 57+57= 1 14 cm2.