The circumference of the bottom of a cylindrical paper tube is 9 cm, and its side is unfolded to form a square. What is the side area of this cylindrical paper tube in square centimeters?
一个圆柱形纸筒的底面周长是9厘米，把它的侧面展开后是一个正方形，这个圆柱形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>