circular piece of iron has an outer radius of 3 decimeters and an inner diameter of 4 decimeters. What is the area of this circular iron piece in square decimeters?
一个圆形铁片，外圆半径是3分米，内圆直径是4分米，这个环形铁片的面积是多少平方分米？
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">3.14 * 5</gadget>
<output>15.7</output>

<result>15.7</result>