Cross ratio and element at infinity are two important concepts with projective geometry and primary tools in considerate projective property of figures.
交比和无穷远元素是射影几何中的两个重要概念，它们是探讨图形射影性质的主要工具。
Two different surface straight lines distance not essential discovers male hangs the line segment, only must solve using the vector projective property then.
两条异面直线的距离也不必要找出公垂线段，只需利用向量的射影性质来解即可。
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