Use a piece of rectangular paper with a length of 31.4 cm and a width of 12.56 cm to just enclose the side of a cylinder. What is the side area of the cylinder?
用一张长31.4厘米，宽12.56厘米的长方形纸，正好把一个圆柱的侧面围严，这个圆柱的侧面积是多少．
<gadget id="calculator">31.4 * 12.56</gadget>
<output>394.384</output>

<result>394.384</result>