tetrahedron of reference

例句：

1. 1.The tetrahedron is a shape with four equal sides and four right angles.
   正四面体是一种有四个相等的边和四个直角的形状。
2. 2. The tetrahedron is used in geometry to represent the shape of a regular polygon.
   在几何学中，正四面体用来表示一个规则多边形的形状。
3. 3.The tetrahedron has been used as a model for studying the properties of crystals.
   正四面体被用作研究晶体性质的模型。