The radius of the inner circle of a circle is 2 cm, and the diameter of the outer circle is 8 cm. What is the area of the circle in square centimetres?
一个环形内圆半径是2cm，外圆的直径是8cm，这个环形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">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">3.14 * 12</gadget>
<output>37.68</output>

<result>37.68</result>