The diameter of the bottom surface of a cylindrical ventilation pipe is 20 cm, and the length is 50 cm. How many square centimeters of iron sheet should be used to make the ventilation pipe?
圆柱形通风管的底面直径是20厘米，长50厘米，制通风管要用铁皮多少平方厘米．
<gadget id="calculator">3.14 * 20 * 50</gadget>
<output>3_140</output>

<result>3_140</result>