A ring has an outer diameter of 10 cm and an inner radius of 3 cm. What is the area of the ring?
一个圆环，外圆直径是10厘米，内圆半径是3厘米，圆环的面积是多少．
<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<result>50.24</result>