Developed Materials in the Virtual Mathematics Laboratory http://www.math.bas.bg/omi/cabinet/content/bg/html/d22130.html http://www.math.bas.bg/omi/cabinet/content/bg/html/d22130.html http://www.math.bas.bg/omi/cabinet/content/bg/html/d22400.html http://www.math.bas.bg/omi/cabinet/content/bg/html/d22401.html