If a cow is tied to a stake with a rope 3 meters long, what is the maximum area in square meters that the cow grazes?
把一头牛用3米长的绳系在一根木桩上，这头牛吃草的最大面积是多少平方米．
<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

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

<result>28.26</result>