有四个等边三角形面的四面体。

A tetrahedron with four equilateral triangular faces.

youdao

因此在空间中是一个三角形，空间中是一个四面体，等等。

Thus one has a triangle in, a tetrahedron in, etc.

youdao

一个完满的理论必须能够解释甲烷中为什么所有hch角都具有四面体的特性值。

A satisfactory theory must be able to explain why in methane the HCH angles all have the characteristic tetrahedral value.

youdao

And so now I have hydrogens at the four corners of a tetrahedron.

放在碳正四面体,的四个角上。

麻省理工公开课 - 固态化学导论课程节选