A circular machine part, the diameter of the outer ring is 8 cm, and the diameter of the inner ring is 6 cm, what is the area of the ring part in square centimeters?
一个环形机器零件，外环直径是8厘米，内环直径是6厘米，这个环形零件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">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>