Date: Dec 28, 1997 3:26 AM Author: Michael Keyton Subject: Re: Orthocentre The orthocenter is one of the more important points of a triangle. Of the

4 major centers, it is the only one not mentioned in Euclid.

One could write a book about this point, there are so many interesting

aspects of it. I taught a class on euclidean geoemtry for high school

teachers last year, and opened each class with a property of the

orthocenter, and I just scratched the surface.

just a few.

Possibly the most important aspect: take a triangle with vertices at T, R,

and I, and let H be its orthocenter. The orthocenter for any of the

triangles formed from three of these four points is the fourth point. (4

points which satisfy this condition are called an orthocentric set.)

Bisect the exterior angles of a triangle, they intersect at three points,

called the excenters. The incenter of the triangle is the orthocenter of

the triangle formed by the 3 excenters. Thus, the incenter and the 3

excenters form an orthocentric set. (There are many other orthocentric

sets.)

From this it then follows: let O, C, and E be the feet of the altitudes of

triangle TRI. Triangle OCE is called the orthic triangle of TRI, the

altitudes bisect the angles of the orthic triangle; i.e., the incenter of

the orthic triangle is the orthocenter of the original tirangle.

The orthocenter, the centroid, and the circumcenter are collinear, the

centroid is a trisection point. The line through these 3 points is called

the Euler line. (There are many proofs of this, some surprisingly quite

simple, one using dilations makes the theorem obvious (both aspects).

The circumcircle of the orthic triangle contains the midpoints of the

sides of the triangle, and the midpoints from the othocenter to each of

the vertices of the triangle. (This circle is called the 9 point circle,

though, there are now known over 40 points that reside on it.) (It is also

called the Feuerbach circle. It is tangent to the incircle and the three

excircles.) The center of this circle is the midpoint between the

orthocenter and the circumcenter; it is called the 9 point circle center.)

The distance from the orthocenter to a vertex is half the distance from

the circumcenter to the opposite side of the vertex.

You mentioned reflecting the orthocenter over the sides of the triangle

proces points that live on the circumcircle of the triangle.

Reverse this process, (which is very impressive on Cabri) take any point

on the circumcircle, reflect it over the sides of the triangle. They are

collinear. Take the locus of these points as the point moves about the

circumcircle. These describe three circles which are concurrent at the

orthocenter of the triangle.

This is a few of the interesting aspects of the orthocenter. If you are

interested in others, I can supply you with several excellent references.

Michael Keyton

On 27 Dec 1997, Hai Trung Ho wrote:

> I only know that the orthocentre, reflected on one of the triangle's

> sides will lie on the circumcircle of the triangle. Are there any

> other properties of the orthocentre???

>

> Hai Trung Ho.

>