A ring, the diameter of the big circle is 8cm, and the radius of the small circle is 3cm, what is the area of the ring in cm**2.
一个圆环，大圆直径8cm，小圆半径3cm，这个圆环的面积是多少cm**2．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

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

<gadget id="calculator">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>