Geodesic
On inequalities in a tetrahedron, finished
Matematičeskoe obrazovanie, Tome 99 (2021) no. 3, pp. 58-69.

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. 
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V. N. Novikov. On inequalities in a tetrahedron, finished. Matematičeskoe obrazovanie, Tome 99 (2021) no. 3, pp. 58-69. http://geodesic.mathdoc.fr/item/MO_2021_99_3_a6/

[1] D. S. Mitrinovic, J. I. Pecaric, V. Volenec, Recent advances in geometric inequalities, Kluwer Academic Publishers, Dordrecht-Boston-London, 1989 <ext-link ext-link-type='zbl-item-id' href='https://zbmath.org/?q=an:0679.51004'>0679.51004</ext-link>

[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