Two concentric circles with different radii have an inner radius of 3 cm and an outer diameter of 8 cm. What is the area of the rings in square centimeters?
两个半径不同的同心圆，内半径3厘米，外直径8厘米，圆环的面积是多少平方厘米．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<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>