Herr is an exercise from basic geometry in space. It refers to a question on Pierre Lecomte's Blog and the ensuing discussion.

The methan molekule CH4 is a regular tetrahedron with the carbon atom in the center. Can you determine, with a minimum of calculation, the angle between two carbon-hydrogen bonds?
comment résoudre une équation avec des puissances

In fact this is a classical exercice for chemists. I have numbered the hydrogen atoms H1, H2, H3 and H4 to fix the notations for your calculations.