To make a ventilating pipe with a bottom diameter of 30cm and a length of 40cm, how much iron sheet is needed at least in cm**2.
做一节底面直径是30cm，长40cm的通风管，至少需要铁皮多少cm**2．
<gadget id="calculator">3.14 * 30 * 40</gadget>
<output>3_768</output>

<result>3_768</result>