arXiv:1011.1236v1; This article is a translation of the original (in Polish), written for Matematyka, Spoleczenstwo, Nauczanie, Vol.46. It is an extended version of the talk by the author at the XLV Szkola Matematyki Pogladowej, Jachranka, Poland, 27-31 August 2010, see http://www.msn.ap.siedlce.pl