When deriving the formula for calculating the area of a circle, divide a circle into several equal parts and put together an approximate rectangle; measure the width of the rectangle to be 3 centimeters, what is the area of the circle in square centimeters.
在推导圆面积计算公式时，将一个圆平均分成若干等份，拼成一个近似的长方形；量得长方形宽3厘米，这个圆的面积是多少平方厘米．
<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

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

<result>28.26</result>