To make a cylindrical ventilation pipe with a bottom radius of 4dm and a length of 10dm, at least how many square decimeters of iron sheet are needed.
做一节底面半径4dm，长10dm的圆柱形通风管，至少需要铁皮多少平方分米．
<gadget id="calculator">2 * 3.14 * 4 * 10</gadget>
<output>251.2</output>

<result>251.2</result>