2.14. Calculate the dihedral angle cosine of a regular tetrahedron.
2.14. Let $\varphi$ be the dihedral angle along the edge of a regular tetrahedron; $O$ - the projection of the vertex $D$ of a regular tetrahedron $A B C D$ onto the opposite face. Then $\cos \varphi = S_{A B O} : S_{A B D} = 1 / 3$