Geodesic
On inequalities in a tetrahedron
Matematičeskoe obrazovanie, no. 2 (2021), pp. 18-27 
V. N. Novikov. On inequalities in a tetrahedron. Matematičeskoe obrazovanie, no. 2 (2021), pp. 18-27. http://geodesic.mathdoc.fr/item/MO_2021_2_a2/
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     author = {V. N. Novikov},
     title = {On inequalities in a tetrahedron},
     journal = {Matemati\v{c}eskoe obrazovanie},
     pages = {18--27},
     year = {2021},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MO_2021_2_a2/}
}
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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. To be continued.