From all theorems of elementary geometry in school there are many important theorems as Pythagorean Theorem and law of sins, and there are many theorems that can evoke total ignorance e.g. 1. If in right-angled triangle is drawn an altitude to the hypotenuse, then square of cachets is middle-proportional between hypotenuse and this cachets projection on hypotenuse. 2. There is Ptolemy’s theory about inscribed quadrilateral in circle. If we draw circle and inscribe quadrilateral in it then in inscribed quadrilateral product segments of diagonals are equal. For these theorems still haven’t find applications.