Serbia. Knjazevac is a town and municipality located in the Zaječar District of the eastern Serbia. « Vuk Karadzic » Primary School. Classroom. 8th Grade. Student during mathematics class. Formulas, mathematical relationships and geometric figures. The number π is a mathematical constant. Originally defined as the ratio of a circle's circumference to its diameter, it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics. It is approximately equal to 3.14159. The Pestalozzi Children's Foundation (Stiftung Kinderdorf Pestalozzi) is advocating access to high quality education for underprivileged children. It supports in Knjazevac a project called " Education for child rights" 20.4.2018 © 2018 Didier Ruef for the Pestalozzi Children's Foundation