Geodesic
A Sum of Angles of a Self-intersecting Polygon
Matematičeskoe obrazovanie, Tome 28 (2004) no. 1, pp. 78-86.

Voir la notice de l'article provenant de la source Math-Net.Ru

The problem is supposed to be investigated by school students which start to study geometry. 
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A. I. Schetnikov. A Sum of Angles of a Self-intersecting Polygon. Matematičeskoe obrazovanie, Tome 28 (2004) no. 1, pp. 78-86. http://geodesic.mathdoc.fr/item/MO_2004_28_1_a2/