The circumference of a circle is 31.4 decimeters, what is the area of the circle in square decimeters?
一个圆的周长是31.4分米，这个圆的面积是多少平方分米．
<gadget id="calculator">2 * 3.14</gadget>
<output>6.28</output>

<gadget id="calculator">31.4 / 6.28</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>