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在平面几何中我们学过这样一个性质:如果一条直线经过一条线段的中点,那么这条线段的两个端点到这条直线的距离相等(如图1 所示)．现在我们把这条性质拓展延伸到立体几何中,则有:如果一个平面经过一条线段的中
In planar geometry we have learned a property that if a straight line passes through the midpoint of a line segment, then the two endpoints of the line segment have equal distances from this line (as shown in Figure 1). Now we extend this property extension to the three-dimensional geometry. If there is a plane passing through a line segment,