The outer radius of a ring is 0.4 meters and the inner radius is 0.3 meters. What is the area of the ring in square meters?
一个环形的外圆半径是0.4米，内圆半径是0.3米，这个环形的面积是多少平方米．
<gadget id="calculator">0.4 ** 2</gadget>
<output>0.16</output>

<gadget id="calculator">3.14 * 0.16</gadget>
<output>0.5024</output>

<gadget id="calculator">0.3 ** 2</gadget>
<output>0.09</output>

<gadget id="calculator">3.14 * 0.09</gadget>
<output>0.2826</output>

<gadget id="calculator">0.5024 - 0.2826</gadget>
<output>0.2198</output>

<result>0.2198</result>