Known shadow area is 20 square decimeters, ring area

Let R be the radius of the big circle and R be the radius of the small circle. According to the meaning of the question, r2-r2 = 20 (square decimeter), so the area of the ring is 3.14 × 20 = 62.8 (square decimeter). A: the area of the ring is 62.8 square decimeter

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