Geodesic
On Necessity of Argument while Solving Stereometric Problems
Matematičeskoe obrazovanie, Tome 52 (2009) no. 4, pp. 11-18.

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. 
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E. Potoskuev. On Necessity of Argument while Solving Stereometric Problems. Matematičeskoe obrazovanie, Tome 52 (2009) no. 4, pp. 11-18. http://geodesic.mathdoc.fr/item/MO_2009_52_4_a1/