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试着在纸上画一条直线,是不是不太容易画得直?画直线一般要借助于直尺来画。直线有水平的、垂直的和斜的,如左图所示。你如果有一把很长很长的尺子,就可以画出一条很长的直线,它超过纸张、迈出房屋、离开地球,一直延伸到太空。虽然你看不到这条直线能走多远,但它是不断延伸下去的。其实,所有的直线都是没有终点的,我们平常所看到的只是直线的一段而已。为了表示直线是向两边不断延伸的,我们就在线的两端各加一个箭头,如下图所示。
Try to draw a straight line on the paper, is not it easy to draw straight? Draw a straight line to draw on the ruler. Straight lines are horizontal, vertical and oblique, as shown on the left. If you have a very long, long ruler, you can draw a long straight line that extends beyond the paper, into the house, away from the earth, and into space. Although you can not see how far this line can go, it is constantly extending. In fact, all the straight lines have no end, we usually see only a straight line only. In order to show that the line is continuously extended to both sides, we add an arrow at each end of the line, as shown in the following figure.