逆定理:两个三角形的对应边的交点共线,则它们的对应顶点的连线共点. 设两个三角形ABC,A′B′C′中,BC∩B′C′=P,CA∩C′A′=Q,AB∩A′B′=R,且P∪Q∪R,则AA′∩BB′∩CC′(=O). 换言之,即两个三角形如成轴透视,便成中心透视.以符号表示之,即△ABC l The opposite theorem: The intersections of the corresponding sides of the two triangles are collinear, and their corresponding vertices are connected together. Set two triangles ABC, A’B’C’, BC∩B’C’=P,CA∩ C′A′=Q, AB∩A′B′=R, and P∪Q∪R, then AA′∩BB′∩CC′(=O). In other words, that is, if the two triangles form a perspective, they become Center perspective. Symbolically, △ABC l