ring has an outer radius of 5 cm and an inner radius of 4 cm. What is the area of the ring in cm**2.
一个环形，外圆半径为5cm，内圆半径为4cm，环形的面积为多少cm**2．
<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>