Geodesic
On Necessity of Argument while Solving Stereometric Problems
Matematičeskoe obrazovanie, no. 4 (2009), pp. 11-18 
E. Potoskuev. On Necessity of Argument while Solving Stereometric Problems. Matematičeskoe obrazovanie, no. 4 (2009), pp. 11-18. http://geodesic.mathdoc.fr/item/MO_2009_4_a1/
@article{MO_2009_4_a1,
     author = {E. Potoskuev},
     title = {On {Necessity} of {Argument} while {Solving} {Stereometric} {Problems}},
     journal = {Matemati\v{c}eskoe obrazovanie},
     pages = {11--18},
     year = {2009},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MO_2009_4_a1/}
}
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Voir la notice de l'article provenant de la source Math-Net.Ru

Argument is still necessary if some stereometric fact seems to be "intuitively clear" or "evident". This is illustrated by numerous examples.