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