# Nine-point center Information

*https://en.wikipedia.org/wiki/Nine-point_center*

In
geometry, the **nine-point center** is a
triangle center, a point defined from a given
triangle in a way that does not depend on the placement or scale of the triangle.
It is so called because it is the center of the
nine-point circle, a circle that passes through nine significant points of the triangle: the midpoints of the three edges, the feet of the three
altitudes, and the points halfway between the
orthocenter and each of the three vertices. The nine-point center is listed as point X(5) in
Clark Kimberling's
Encyclopedia of Triangle Centers.^{
[1]}^{
[2]}

## Properties

The nine-point center *N* lies on the
Euler line of its triangle, at the
midpoint between that triangle's
orthocenter *H* and
circumcenter *O*. The
centroid *G* also lies on the same line, 2/3 of the way from the orthocenter to the circumcenter,^{
[2]}^{
[3]} so

Thus, if any two of these four triangle centers are known, the positions of the other two may be determined from them.

Andrew Guinand proved in 1984, as part of what is now known as **Euler's triangle determination problem**, that if the positions of these centers are given for an unknown triangle, then the
incenter of the triangle lies within the
orthocentroidal circle (the circle having the segment from the centroid to the orthocenter as its diameter). The only point inside this circle that cannot be the incenter is the nine-point center, and every other interior point of the circle is the incenter of a unique triangle.^{
[4]}^{
[5]}^{
[6]}^{
[7]}

The distance from the nine-point center to the
incenter *I* satisfies

where *R* and *r* are the
circumradius and
inradius respectively.

The nine-point center is the
circumcenter of the
medial triangle of the given triangle, the circumcenter of the
orthic triangle of the given triangle, and the circumcenter of the Euler triangle.^{
[3]} More generally it is the circumcenter of any triangle defined from three of the nine points defining the nine-point circle.

The nine-point center lies at the
centroid of four points: the triangle's three vertices and its
orthocenter.^{
[8]}

The
Euler lines of the four triangles formed by an
orthocentric system (a set of four points such that each is the
orthocenter of the triangle with vertices at the other three points) are
concurrent at the nine-point center common to all of the triangles.^{
[9]}^{:p.111}

Of the nine points defining the nine-point circle, the three midpoints of line segments between the vertices and the orthocenter are reflections of the triangle's midpoints about its nine-point center. Thus, the nine-point center forms the center of a
point reflection that maps the medial triangle to the Euler triangle, and vice versa.^{
[3]}

According to
Lester's theorem, the nine-point center lies on a common circle with three other points: the two
Fermat points and the circumcenter.^{
[10]}

The
Kosnita point of a triangle, a triangle center associated with
Kosnita's theorem, is the
isogonal conjugate of the nine-point center.^{
[11]}

## Coordinates

Trilinear coordinates for the nine-point center are^{
[1]}^{
[2]}

The
barycentric coordinates of the nine-point center are^{
[2]}

Thus if and only if two of the vertex angles differ from each other by more than 90°, one of the barycentric coordinates is negative and so the nine-point center is outside the triangle.

## References

- ^
^{a}^{b}Kimberling, Clark (1994), "Central Points and Central Lines in the Plane of a Triangle",*Mathematics Magazine*,**67**(3): 163–187, doi: 10.2307/2690608, JSTOR 2690608, MR 1573021. - ^
^{a}^{b}^{c}^{d}Encyclopedia of Triangle Centers, accessed 2014-10-23. - ^
^{a}^{b}^{c}Dekov, Deko (2007), "Nine-point center" (PDF),*Journal of Computer-Generated Euclidean Geometry*. **^**Stern, Joseph (2007), "Euler's triangle determination problem" (PDF),*Forum Geometricorum*,**7**: 1–9.**^**Euler, Leonhard (1767), "Solutio facilis problematum quorundam geometricorum difficillimorum",*Novi Commentarii Academiae Scientiarum Petropolitanae*(in Latin),**11**: 103–123.**^**Guinand, Andrew P. (1984), "Euler lines, tritangent centers, and their triangles",*American Mathematical Monthly*,**91**(5): 290–300, doi: 10.2307/2322671, JSTOR 2322671.**^**Franzsen, William N. "The distance from the incenter to the Euler line",*Forum Geometricorum*11, 2011, 231-236. http://forumgeom.fau.edu/FG2011volume11/FG201126index.html**^**The Encyclopedia of Triangle Centers credits this observation to Randy Hutson, 2011.**^**Altshiller-Court, Nathan,*College Geometry*, Dover Publications, 2007 (orig. Barnes & Noble 1952).**^**Yiu, Paul (2010), "The circles of Lester, Evans, Parry, and their generalizations",*Forum Geometricorum*,**10**: 175–209, MR 2868943.**^**Rigby, John (1997), "Brief notes on some forgotten geometrical theorems",*Mathematics and Informatics Quarterly*,**7**: 156–158.