A circular washer, the diameter of the inner circle is 6 cm, the diameter of the outer circle is 8 cm, what is the area of the washer in square centimeters?
一个环形垫圈，内圆的直径是6厘米，外圆的直径是8厘米，垫圈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>