In a square with a circumference of 40 decimeters, construct the largest circle, what is the area of this circle in square decimeters?
在一个周长是40分米的正方形内，作一个最大的圆，这个圆的面积是多少平方分米？
<gadget id="calculator">40 / 4 / 2</gadget>
<output>5</output>

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

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

<result>78.5</result>