A circle has an outer radius of 9 cm and an inner radius of 5 cm. Find the area of this circle.
一个圆环形，外圆半径是9cm，内圆半径是5cm．求这个环形的面积．
<gadget id="calculator">9 ** 2</gadget>
<output>81</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">81 - 25</gadget>
<output>56</output>

<gadget id="calculator">3.14 * 56</gadget>
<output>175.84</output>

<result>175.84</result>