Use a 4-meter-long rope to wrap around the trunk of a big tree, leaving 0.86 meters left. What is the cross-sectional area of the big tree in square meters?
用一条4米长的绳子，绕一棵大树树干一圈，还余0.86米，这棵大树干的横截面面积是多少平方米？
<gadget id="calculator">4 - 0.86</gadget>
<output>3.14</output>

<gadget id="calculator">3.14 / 3.14 / 2</gadget>
<output>0.5</output>

<gadget id="calculator">3.14 * 0.5 * 0.5</gadget>
<output>0.785</output>

<result>0.785</result>