A ring has an outer diameter of 8 cm and an inner radius of 30 cm. Find the area of this circle?
一个圆环的外圆直径是8分米，内圆半径是30厘米．求这个圆环的面积？
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

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

<gadget id="calculator">30 / 10</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>