Isoperimetric point explained

In geometry, the isoperimetric point is a triangle center - a special point associated with a plane triangle. The term was originally introduced by G.R. Veldkamp in a paper published in the American Mathematical Monthly in 1985 to denote a point in the plane of a triangle having the property that the triangles have isoperimeters, that is, having the property that^{[1] [2]}

Isoperimetric points in the sense of Veldkamp exist only for triangles satisfying certain conditions. The isoperimetric point of in the sense of Veldkamp, if it exists, has the following trilinear coordinates.^[3]

$$\backslash sec\backslash tfrac\; \backslash cos\backslash tfrac\; \backslash cos\backslash tfrac\; -\; 1\; \backslash \; :\; \backslash \; \backslash sec\backslash tfrac\; \backslash cos\backslash tfrac\; \backslash cos\backslash tfrac\; -\; 1\; \backslash \; :\; \backslash \; \backslash sec\backslash tfrac\; \backslash cos\backslash tfrac\; \backslash cos\backslash tfrac\; -\; 1$$

Given any triangle one can associate with it a point having trilinear coordinates as given above. This point is a triangle center and in Clark Kimberling's Encyclopedia of Triangle Centers (ETC) it is called the isoperimetric point of the triangle . It is designated as the triangle center X(175). The point X(175) need not be an isoperimetric point of triangle in the sense of Veldkamp. However, if isoperimetric point of triangle in the sense of Veldkamp exists, then it would be identical to the point X(175).

The point with the property that the triangles have equal perimeters has been studied as early as 1890 in an article by Emile Lemoine.^{[4] [5]}

Existence of isoperimetric point in the sense of Veldkamp

Let be any triangle. Let the sidelengths of this triangle be . Let its circumradius be and inradius be . The necessary and sufficient condition for the existence of an isoperimetric point in the sense of Veldkamp can be stated as follows.^[1]

The triangle has an isoperimetric point in the sense of Veldkamp if and only if $$a\; +\; b\; +\; c\; >\; 4R\; +\; r.$$

For all acute angled triangles we have, and so all acute angled triangles have isoperimetric points in the sense of Veldkamp.

Properties

Let denote the triangle center X(175) of triangle .^[4]

• lies on the line joining the incenter and the Gergonne point of .
• If is an isoperimetric point of in the sense of Veldkamp, then the excircles of triangles are pairwise tangent to one another and is their radical center.
• If is an isoperimetric point of in the sense of Veldkamp, then the perimeters of are equal to

$$\backslash frac$$where is the area, is the circumradius, is the inradius, and are the sidelengths of .

Soddy circles

Given a triangle one can draw circles in the plane of with centers at such that they are tangent to each other externally. In general, one can draw two new circles such that each of them is tangential to the three circles with as centers. (One of the circles may degenerate into a straight line.) These circles are the Soddy circles of . The circle with the smaller radius is the inner Soddy circle and its center is called the inner Soddy point or inner Soddy center of . The circle with the larger radius is the outer Soddy circle and its center is called the outer Soddy point or outer Soddy center of triangle .^{[6] [7]}

The triangle center X(175), the isoperimetric point in the sense of Kimberling, is the outer Soddy point of .

External links

• isoperimetric and equal detour points - interactive illustration on Geogebratube

Notes and References

1. G. R. Veldkamp. The isoperimetric point and the point(s) of equal detour. Amer. Math. Monthly. 1985. 92. 8. 546–558. 10.2307/2323159. 2323159.
2. Hajja. Mowaffaq. Yff, Peter. The isoperimetric point and the point(s) of equal detour in a triangle. Journal of Geometry. 2007. 87. 1–2. 76–82. 10.1007/s00022-007-1906-y. 122898960.
3. Web site: Kimberling. Clark. Isoperimetric Point and Equal Detour Point. 27 May 2012.
4. Web site: Kimberling . Clark . X(175) Isoperimetric Point . 27 May 2012 . dead . https://web.archive.org/web/20120419171900/http://faculty.evansville.edu/ck6/encyclopedia/ETC.html . 19 April 2012 .
5. The article by Emile Lemoine can be accessed in Gallica. The paper begins at page 111 and the point is discussed in page 126.Gallica
6. Nikolaos Dergiades. The Soddy Circles. Forum Geometricorum. 2007. 7. 191–197. 29 May 2012.
7. Web site: Soddy Circles. 29 May 2012.