Make 20 joints of bottom surface radius and be 5 centimeters, be long the cylindrical iron sheet ventilation pipe of 4 decimeters, how much area iron sheet will be used at least?
制20节底面半径为5厘米、长为4分米的圆柱形铁皮通风管，至少要用多大面积的铁皮？
<gadget id="calculator">5 / 10</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">3.14 * (1/2) * 4 * 20</gadget>
<output>125.6</output>

<result>125.6</result>