立体几何是平面几何的延伸与拓展,两者之间在不断升维与降维的转化中实现内容的补充和问题的解决.虽然有的平面几何定理不能移到空间,但在空间的任一平面上,平面几何的结论都是成立的.因此选取或构造一个恰当的平面,使问题在这个平面上获得突破性进展,甚至全部解决,是一种自然 Three-dimensional geometry is the extension and expansion of the plane geometry, between the two in the continuous dimensioning and dimension reduction to achieve the content of the supplement and problem solving.Although some of the plane geometry theorem can not be moved to space, but in space On the plane, the conclusion of plane geometry is true, so selecting or constructing an appropriate plane makes the problem a breakthrough in this plane or even solving it all, which is a natural