Euler's theorem in geometry

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In geometry, Euler's theorem states that the distance d between the circumcentre and incentre of a triangle can be expressed as[1][2][3][4]

 d^2=R (R-2r) \,

where R and r denote the circumradius and inradius respectively (the radii of the above two circles). The theorem is named for Leonhard Euler, who published it in 1767.[5] However, the same result was published earlier by William Chapple in 1746.[6]

From the theorem follows the Euler inequality:[2][3]

R \ge 2r.


A figure for following the proof (which also contains the proof here). Made in GeoGebra software.

Let O be the circumcentre of triangle ABC, and I be its incentre, the extension of AI intersects the circumcircle at L, then L is the midpoint of arc BC. Join LO and extend it so that it intersects the circumcircle at M. From I construct a perpendicular to AB, and let D be its foot, then ID = r. It is not difficult to prove that triangle ADI is similar to triangle MBL, so ID / BL = AI / ML, i.e. ID × ML = AI × BL. Therefore 2Rr = AI × BL. Join BI, because

angle BIL = angle A / 2 + angle ABC / 2,
angle IBL = angle ABC / 2 + angle CBL = angle ABC / 2 + angle A / 2,

therefore angle BIL = angle IBL, so BL = IL, and AI × IL = 2Rr. Extend OI so that it intersects the circumcircle at P and Q, then PI × QI = AI × IL = 2Rr, so (R + d)(R − d) = 2Rr, i.e. d2 = R(R − 2r).

Stronger version of the inequality[edit]

A stronger version[7] is

\frac{R}{r} \geq \frac{abc+a^3+b^3+c^3}{2abc} \geq \frac{a}{b}+\frac{b}{c}+\frac{c}{a}-1 \geq \frac{2}{3} \left(\frac{a}{b}+\frac{b}{c}+\frac{c}{a} \right) \geq 2.

See also[edit]


  1. ^ Johnson, Roger A. (2007) [1929], Advanced Euclidean Geometry, Dover Publ., p. 186 .
  2. ^ a b Alsina, Claudi; Nelsen, Roger (2009), When Less is More: Visualizing Basic Inequalities, Dolciani Mathematical Expositions 36, Mathematical Association of America, p. 56, ISBN 9780883853429 .
  3. ^ a b Debnath, Lokenath (2010), The Legacy of Leonhard Euler: A Tricentennial Tribute, World Scientific, p. 124, ISBN 9781848165250 .
  4. ^ Dunham, William (2007), The Genius of Euler: Reflections on his Life and Work, Spectrum Series 2, Mathematical Association of America, p. 300, ISBN 9780883855584 .
  5. ^ Euler, Leonhard (1767), Solutio facilis problematum quorumdam geometricorum difficillimorum, Novi Commentarii academiae scientiarum Petropolitanae (in Latin) 11: 103–123 .
  6. ^ 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.
  7. ^ Svrtan, Dragutin; Veljan, Darko (2012), Non-Euclidean versions of some classical triangle inequalities, Forum Geometricorum 12: 197–209 . See in particular p. 198.

External links[edit]