Fagnano's Problem

In geometry, Fagnano's problem is an optimization problem that was first stated by Giovanni Fagnano in 1775:

The solution is the orthic triangle, with vertices at the base points of the altitudes of the given triangle.

Solution

The orthic triangle, with vertices at the base points of the altitudes of the given triangle, has the smallest perimeter of all triangles inscribed into an acute triangle, hence it is the solution of Fagnano's problem. Fagnano's original proof used calculus methods and an intermediate result given by his father Giulio Carlo de' Toschi di Fagnano. Later however several geometric proofs were discovered as well, amongst others by Hermann Schwarz and Lipót Fejér. These proofs use the geometrical properties of reflections to determine some minimal path representing the perimeter.

Physical principles

A solution from physics is found by imagining putting a rubber band that follows Hooke's law around the three sides of a triangular frame $ABC$, such that it could slide around smoothly. Then the rubber band would end up in a position that minimizes its elastic energy, and therefore minimize its total length. This position gives the minimal perimeter triangle.
The tension inside the rubber band is the same everywhere in the rubber band, so in its resting position, we have, by Lami's theorem, $\angle bcA = \angle acB, \angle caB = \angle baC, \angle abC = \angle cbA$

Therefore, this minimal triangle is the orthic triangle.

Proofs in absolute geometries

The minimality of the perimeter of the orthic triangle can be proven in a more general setting, that of absolute geometry and even weaker settings.

See also

*Set TSP problem, a more general task of visiting each of a family of sets by the shortest tour

References

*
*Paul J. Nahin: ''When Least is Best: How Mathematicians Discovered Many Clever Ways to Make Things as Small (or as Large) as Possible''. Princeton University Press 2004, , p. 67
* Coxeter, H. S. M.; Greitzer, S. L.:''Geometry Revisited''. Washington, DC: Math. Assoc. Amer. 1967, pp. 88–89.
* H.A. Schwarz: ''Gesammelte Mathematische Abhandlungen, vol. 2''. Berlin 1890, pp. 344–345.
online
at the Internet Archive, German)
*

References

External links

Fagnano's problem at cut-the-knotFagnano's problem
in the Encyclopaedia of Mathematics
*
*

{{DEFAULTSORT:Fagnano'S Problem
Triangle problems