A ring has an inner diameter of 6 cm and an outer diameter of 8 cm, what is its area in cm**2.
一个圆环，内直径是6cm，外直径是8cm，它的面积是多少cm**2．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</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>