How many square decimeters of iron sheet are required to make a cylindrical iron sheet ventilation pipe with a length of 4 meters and a bottom radius of 5 cm?
制一节长4米，底面半径是5厘米的圆柱形铁皮通风管，至少需多少平方分米的铁皮．
<gadget id="calculator">5 / 10</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 * 10</gadget>
<output>40</output>

<gadget id="calculator">3.14 * (1/2) * 2 * 40</gadget>
<output>125.6</output>

<result>125.6</result>