Use a square piece of paper with an area of 40 square centimeters to cut out the largest circle. What is the area of this circle in square centimeters?
用一张面积为40平方厘米的正方形纸片，剪下一个最大的圆，这个圆的面积是多少平方厘米．
<gadget id="calculator">3.14 * 10</gadget>
<output>31.4</output>

<result>31.4</result>