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In 2009, Putuo District primary school mathematics grade six graduation quality examination paper.
The sum of radii is 3+1= 4cm.

So the distance traveled by the center of the small circle is 3. 14×2×4=25. 12 cm.

The area is the area of this black ring.

Wherein the radius of the outer ring is 3+1× 2 = 5cm.

So 3.14× 5× 5-3.14× 3× 3 = 50.24 square centimeters.