各个面都是直角三角形的四面体是一个简单的几何体 ,我们把它称之为基本体 .立体几何中很多重要的概念和定理 ,如三垂线定理 ,线面、面面平行与垂直的概念与性质定理 ,各种角与距离、体积的概念和计算等都能从这个基本体中衍生 ,这正是近十几年来全国高考数学试题关注这个基本 A tetrahedron whose face is a right-angled triangle is a simple geometry. We call it a primitive. Many important concepts and theorems in solid geometry, such as the triple perpendicular theorem, the concepts of line faces, faces parallel and vertical And the theorem of nature, the concept and calculation of various angles and distances, volumes, etc. can all be derived from this basic body. This is exactly the basics of the national college entrance examination mathematics test questions in the past ten years.