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。
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。
Cross ratio and element at infinity are two important concepts with projective geometry and primary tools in considerate projective property of figures.
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