1问题大家知道,对于一个三棱锥,三条侧棱相等(或三侧棱与底面成等角)是顶点在底面上的射影为底面外心的充要条件;三侧面与底面成等角(或三顶点到底面三边等距)是顶点在底面上的射影 1 Problem We all know that for a triangular pyramid, the three side edges are equal (or the three side edges are at equal angles to the bottom surface) is the necessary and sufficient condition that the vertex projected on the bottom surface is the outer center of the bottom surface; the three sides are equal to the bottom surface (or Three vertices to the bottom three sides are equally spaced) is the projection of the vertex on the bottom surface