A circular iron plate has an outer diameter of 9 meters and an inner diameter of 5 meters. What is the area of half of this iron plate?
一块环行铁片，外圆直径9米，内圆直径5米，求这个铁片一半的面积是多少？
<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">(9/2) ** 2</gadget>
<output>81/4 = around 20.25</output>

<gadget id="calculator">3.14 * (81/4)</gadget>
<output>63.585</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">(5/2) ** 2</gadget>
<output>25/4 = around 6.25</output>

<gadget id="calculator">3.14 * (25/4)</gadget>
<output>19.625</output>

<gadget id="calculator">63.585 - 19.625</gadget>
<output>43.96</output>

<gadget id="calculator">43.96 / 2</gadget>
<output>21.98</output>

<result>21.98</result>