Cut a circle along the radius and assemble it into an approximate rectangle. If the length of the rectangle is 15.7m decimeters, what is the area of the circle in square decimeters?
把一个圆沿半径剪开，拼成一个近似的长方形，如果长方形的长是15.7m分米，那么这个圆的面积是多少平方分米？
<gadget id="calculator">15.7 / 3.14</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>