A circular piece of iron has an outer diameter of 10 decimeters and an inner radius of 3 decimeters. What is its area.
一块环形铁片，外圆直径是10分米，内圆半径是3分米．它的面积是多少．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

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

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

<gadget id="calculator">78.5 - 28.26</gadget>
<output>50.24</output>

<result>50.24</result>