A ring has an inner radius of 3 cm and a ring width of 1 cm. What is the area of the circle in square centimetres.
一个圆环的内圆半径是3厘米，环宽1厘米．这个圆环的面积是多少平方厘米．
<gadget id="calculator">3 + 1</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>