A circular iron plate has an outer diameter of 6 cm and an inner radius of 2 cm. What is the area of the iron plate in square centimetres?
一个圆环形铁片，外直径是6厘米，内半径是2厘米，这个铁片的面积是多少平方厘米．
<gadget id="calculator">6 / 2</gadget>
<output>3</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>