A web page and interactive applet demonstrating the concept of the orthocenter of a triangle.

The applet shows a triangle where the user can drag the vertices around. The orthocenter and the altitude lines that generate it are continuously moved as the user drags the vertices. By experimentation the user can observe that the orthocenter can be inside or outside the triangle.

The page text explains the definition of the orthocenter and how to find it.

Applet can be enlarged to full screen size for use with a classroom projector. After use in the classroom, student can access it again from any web browser at home or in the library with no login required.

This resource is a component of the Math Open Reference Interactive Geometry textbook project at http://www.mathopenref.com.