线段在直线上的正射影xianduan zai zhixian shang dezhengsheying
线段的两个端点在直线上的正射影之间的线段.如图，线段AB在直线MN上的正射影是线段A′B′；线段CD在直线MN上的正射影是CD′；线段EF在直线MN上的正射影是E′F′.
若线段垂直于直线，则这条线段在直线上的正射影“缩成”一个点.如线段HK所在直线与直线MN垂直，线段HK在直线MN上的正射影是点O.
线段在直线上的正射影，可以看作“构成线段的点集”在直线上“正射影的点集”.
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