直线被称为是自叠合的,因为直线的任何一段都能同长度相等的其他任何一段完全叠合。圆的圆周也是这样。圆周的任何部分都同长度相等的其他任何部分完全一样。卵形本不是自叠合的,因为它的各个部分有着不同的曲率。从卵形线侧部取下的部分,不能同其端部更为弯曲的部分相叠合。还有第三种线,也像直线和圆周那样,是自叠合的。你能想象出它是哪一类线吗? Straight lines are said to be self-overlapping, because any section of a straight line can be completely overlapped with any other section of equal length. The same is true of the circle. Any part of the circumference is exactly the same as any other part of the same length. The oval is not self-folding, because of its different parts of the curvature. The part removed from the side of the oval can not be overlapped with the more curved part of the end. There is also a third line, also like a straight line and a circle, is self-overlapping. Can you imagine what kind of line it is?