The inner radius of a circle is 3cm and the outer radius is 4cm. What is the area of the circle?
一个环形的内半径是3cm，外半径是4cm，这个环形的面积是多少．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

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

<gadget id="calculator">16 - 9</gadget>
<output>7</output>

<gadget id="calculator">3.14 * 7</gadget>
<output>21.98</output>

<result>21.98</result>