Geodesic
On inequalities in a tetrahedron, finished
Matematičeskoe obrazovanie, no. 3 (2021), pp. 58-69 
V. N. Novikov. On inequalities in a tetrahedron, finished. Matematičeskoe obrazovanie, no. 3 (2021), pp. 58-69. http://geodesic.mathdoc.fr/item/MO_2021_3_a6/
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     author = {V. N. Novikov},
     title = {On inequalities in a tetrahedron, finished},
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     year = {2021},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MO_2021_3_a6/}
}
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Voir la notice de l'article provenant de la source Math-Net.Ru

The exact boundaries of variation of the surface area, volume, and other quantities of tetrahedrons are found for a given ratio of the radii of the inscribed and circumscribed spheres. The relationship between the method of conditional extremum and envelopes is considered on rather bright and meaningful examples. Finished.

[1] D. S. Mitrinovic, J. I. Pecaric, V. Volenec, Recent advances in geometric inequalities, Kluwer Academic Publishers, Dordrecht-Boston-London, 1989 | Zbl

[2] V. N. Novikov, Formuly v pyli tysyacheletii, “Sherna”, 2020 (Nomer yuridicheskoi registratsii dokumenta (teksta): 50/35-N/50-2020-1-404)

[3] Ya. P. Ponarin, Elementarnaya geometriya, v. 2, Izd. MTsNMO, 2006