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例1 图为一卷紧绕成的牛皮纸,纸卷直径为20厘米,中间有一直径为6厘米的卷轴。已知纸的厚度为0.4毫米,问这卷纸展开后大约有多长?
Example 1 is a tightly wound kraft paper with a roll of 20 cm in diameter and a 6 cm diameter roll in the middle. The thickness of the paper is known to be 0.4 millimeter and asks about how long the roll of paper expands?