In geometry, **Euler's theorem** states that the distance *d* between the circumcenter and incenter of a triangle is given by^{[1]}^{[2]}

^{[3]}However, the same result was published earlier by William Chapple in 1746.

^{[4]}

From the theorem follows the **Euler inequality**:^{[5]}^{[6]}

^{[7]}

## Stronger version of the inequality

A stronger version^{[7]} is

## Euler's theorem for the escribed circle

If and denote respectively the radius of the escribed circle opposite to the vertex and the distance between its center and the center of the circumscribed circle, then .

## Euler's inequality in absolute geometry

Euler's inequality, in the form stating that, for all triangles inscribed in a given circle, the maximum of the radius of the inscribed circle is reached for the equilateral triangle and only for it, is valid in absolute geometry.^{[8]}

## See also

- Fuss' theorem for the relation among the same three variables in bicentric quadrilaterals
- Poncelet's closure theorem, showing that there is an infinity of triangles with the same two circles (and therefore the same
*R*,*r*, and*d*) - List of triangle inequalities

## References

**^**Johnson, Roger A. (2007) [1929],*Advanced Euclidean Geometry*, Dover Publ., p. 186**^**Dunham, William (2007),*The Genius of Euler: Reflections on his Life and Work*, Spectrum Series,**2**, Mathematical Association of America, p. 300, ISBN 9780883855584**^**Leversha, Gerry; Smith, G. C. (November 2007), "Euler and triangle geometry",*The Mathematical Gazette*,**91**(522): 436–452, JSTOR 40378417**^**Chapple, William (1746), "An essay on the properties of triangles inscribed in and circumscribed about two given circles",*Miscellanea Curiosa Mathematica*,**4**: 117–124. The formula for the distance is near the bottom of p.123.**^**Alsina, Claudi; Nelsen, Roger (2009),*When Less is More: Visualizing Basic Inequalities*, Dolciani Mathematical Expositions,**36**, Mathematical Association of America, p. 56, ISBN 9780883853429**^**Debnath, Lokenath (2010),*The Legacy of Leonhard Euler: A Tricentennial Tribute*, World Scientific, p. 124, ISBN 9781848165250- ^
^{a}^{b}Svrtan, Dragutin; Veljan, Darko (2012), "Non-Euclidean versions of some classical triangle inequalities",*Forum Geometricorum*,**12**: 197–209; see p. 198 **^**Pambuccian, Victor; Schacht, Celia (2018), "Euler's inequality in absolute geometry",*Journal of Geometry*, 109 (Art. 8): 1–11, doi:10.1007/s00022-018-0414-6

## External links

Wikimedia Commons has media related to .Euler's theorem in geometry |