A circular iron plate has an outer diameter of 6 decimeters and an inner radius of 2 decimeters, what is its area in square decimeters?
一个圆环形铁片，外圆直径是6分米，内圆半径是2分米，它的面积是多少平方分米．
<gadget id="calculator">3.14 * 5</gadget>
<output>15.7</output>

<result>15.7</result>