空间中证明“点在线上”主要根据立几的公理二。其证法步骤如下: (1)分析出要证的直线是哪两个平面的交线; (2)再证明要证的点是这两个平面的公共点; (3)由立几公理二,点必在线上。例1 三个平面两两相交,有三条交线,若这三条交线两两相交,则三条交线交于一点。分析:证三线共点可转化为证其中两线的 The proof of “point on line” in space is mainly based on the axiom two of Li. The steps of the certification are as follows: (1) Analyze the intersection between the two planes of the straight line to be proved; (2) Prove that the point to be proved is the common point of these two planes; (3) The axiom of Li et al. , points must be online. Example 1 The three planes intersect with each other and there are three intersection lines. If the three intersection lines intersect each other, the three intersection lines intersect at one point. Analysis: The three points of the card can be converted into two lines