To make a cylindrical iron sheet exhaust pipe with a bottom radius of 4dm and a length of 2m, the minimum amount of iron sheet required is dm**2.
制作一节底面半径是4dm，长为2m的圆柱形铁皮排气管，至少需要铁皮多少dm**2．
<gadget id="calculator">2 * 10</gadget>
<output>20</output>

<gadget id="calculator">2 * 3.14 * 4 * 20</gadget>
<output>502.4</output>

<result>502.4</result>