Divide a circle into several equal small sectors on average, and then put together an approximate parallelogram. The perimeter of this parallelogram is 41.4dm. What is the area of the original circle in square decimeters?
将一个圆平均分割成若干个相等的小扇形，然后拼成一个近似的平行四边形，这个平行四边形的周长是41.4dm，原来圆的面积是多少平方分米？
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<result>78.5</result>