A circle has a radius of 3 meters on its outer circle and a diameter of 4 meters on its inner circle. What is the area of the circle?
一个圆环，外圆的半径是3米，内圆的直径是4米，这个圆环的面积是多少．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

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

<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">3.14 * 5</gadget>
<output>15.7</output>

<result>15.7</result>