A circular iron plate has an outer diameter of 8 cm and an inner radius of 3 cm. What is the area of the iron plate in square centimetres?
一个环形铁片，外圆直径是8厘米，内圆半径是3厘米，铁片的面积是多少平方厘米．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

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

<gadget id="calculator">16 - 9</gadget>
<output>7</output>

<gadget id="calculator">3.14 * 7</gadget>
<output>21.98</output>

<result>21.98</result>