Find the area of a ring whose inner radius is 6cm and outer radius is 7cm.
一个环形的内圆半径是6cm，外圆半径是7cm，求它的面积．
<gadget id="calculator">7 ** 2</gadget>
<output>49</output>

<gadget id="calculator">6 ** 2</gadget>
<output>36</output>

<gadget id="calculator">49 - 36</gadget>
<output>13</output>

<gadget id="calculator">3.14 * 13</gadget>
<output>40.82</output>

<result>40.82</result>