A cylindrical tin bucket without a lid has a bottom diameter of 40 cm and a height of 50 cm. How many square centimeters of iron sheet is needed to make such a bucket? (Only columns are not counted.)
一个无盖的圆柱形铁皮水桶，底面直径是40cm，高是50cm．做这样一个水桶，至少需要多少平方厘米的铁皮？(只列式不计算．)
<gadget id="calculator">3.14 * 40 * 50</gadget>
<output>6_280</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 ** 2</gadget>
<output>400</output>

<gadget id="calculator">3.14 * 400</gadget>
<output>1_256</output>

<gadget id="calculator">6_280 + 1_256</gadget>
<output>7_536</output>

<result>7_536</result>