The outer radius of a ring is 9 cm, and the inner radius is 7 cm. What is the approximate area of the ring?
一个圆环的外圆半径是9厘米，内圆半径是7厘米，圆环的面积约是多少平方厘米．
<gadget id="calculator">9 ** 2</gadget>
<output>81</output>

<gadget id="calculator">7 ** 2</gadget>
<output>49</output>

<gadget id="calculator">81 - 49</gadget>
<output>32</output>

<gadget id="calculator">3.14 * 32</gadget>
<output>100.48</output>

<result>100.48</result>