Two concentric circles with different radii, the inner circle has a radius of 4 cm and the outer circle has a diameter of 10 cm, what is the area of the circle?
两个半径不同的同心圆，内圆半径是4厘米，外圆的直径是10厘米，圆环的面积是多少．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<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>