The radius of the inner circle of a circle is 3.5 cm, and the radius of the outer circle is 4.5 cm. What is the area of the circle?
一个环形内圆半径为3.5厘米，外圆半径为4.5厘米，这个环形的面积是多少．
<gadget id="calculator">4.5 ** 2</gadget>
<output>20.25</output>

<gadget id="calculator">3.5 ** 2</gadget>
<output>12.25</output>

<gadget id="calculator">20.25 - 12.25</gadget>
<output>8</output>

<gadget id="calculator">3.14 * 8</gadget>
<output>25.12</output>

<result>25.12</result>