A circle has an inner radius of 3 cm and an outer radius of 4 cm, what is its area in square centimetres?
有个环形，内圆半径是3厘米，外圆半径是4厘米，它的面积是多少平方厘米．
<gadget id="calculator">16 - 9</gadget>
<output>7</output>

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

<result>21.98</result>