A circular iron plate has an outer radius of 3 cm and a ring width of 1 cm. What is the area of the circular iron plate in square centimetres?
一个环形铁片，外圆半径3厘米，环宽1厘米，环形铁片的面积是多少平方厘米．
<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</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>