To make a chimney with a bottom diameter of 10 cm and a length of 40 cm, at least how many square decimeters of iron sheet are needed.
做一节底面直径为10厘米，长40厘米的烟筒，至少需要多少平方分米铁片．
<gadget id="calculator">3.14 * 10 * 40</gadget>
<output>1_256</output>

<gadget id="calculator">1_256 / 100</gadget>
<output>314/25 = around 12.56</output>

<result>314/25 = around 12.56</result>