A circular piece of iron has an inner radius of 4 cm and an outer radius of 5 cm. What is the area of the ring in square centimetres?
一个环形铁片，内圆半径为4厘米，外圆半径为5厘米，这个环形铁片的面积是多少平方厘米．
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<gadget id="calculator">25 - 16</gadget>
<output>9</output>

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

<result>28.26</result>