A circle has an inner radius of 4cm and an outer radius of 5cm. What is the area of the circle?
一个环形，内圆半径是4cm，外圆半径是5cm，这个环形的面积是多少．
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">25 - 16</gadget>
<output>9</output>

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

<result>28.26</result>