To make a ventilation pipe with a bottom radius of 10dm and a length of 50dm, at least how much iron sheet is needed in m**2.
做一节底面半径为10dm，长为50dm的通风管，至少需要铁皮多少m**2．
<gadget id="calculator">50 / 10</gadget>
<output>5</output>

<gadget id="calculator">6.28 * 5</gadget>
<output>31.4</output>

<result>31.4</result>