A piece of square white paper with a side length of 9 cm is surrounded to form a cylindrical paper tube. What is the lateral area of the paper tube in square centimeters?
把一张边长为9厘米的正方形白纸，围成一个圆柱形纸筒，这个纸筒的侧面积是多少平方厘米．
<gadget id="calculator">9 * 9</gadget>
<output>81</output>

<result>81</result>