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The (first) Lemoine circle special circle associated with triangle. It is named after the French Mathematician Émile Lemoine (1840–1912).

Definition[edit]

The parallels to the three triangle sides through the Lemoine point intersect the (extended) triangle sides in six points. Those six points lie on a common circle and this circle is called the (first) Lemoine circle.

Properties[edit]

The center of the Lemoine circle is the midpoint of the line segment connecting the Lemoine point and the center of the triangle's circumcircle. Its radius can be computed as follows:

{\begin{aligned}r_{L}&={\frac {1}{2}}{\sqrt {R^{2}+r_{C}^{2}}}\\&={\frac {1}{2}}R\sec(\omega )\\&={\frac {abc{\sqrt {a^{2}b^{2}+b^{2}c^{2}+a^{2}c^{2}}}}{(a^{2}+b^{2}+c^{2}){\sqrt {(a+b+c)(-a+b+c)(a-b+c)(a+b-c)}}}}\end{aligned}}

In the formulas above $a,b,c$ denote the lengths of the triangle sides, $\omega$ denotes the Brocard angle, $R$ denotes the radius of the circumcircle and $r_{C}$ the radius of the Cosine circle.

the three triangles $\triangle SP_{a}Q_{a}$ , $\triangle SP_{b}Q_{b}$ und $\triangle SP_{c}Q_{c}$ formed with the Lemoine point and two intersection points on a triangle side are similar to the reference triangle $\triangle ABC$ . The line segments $P_{b}Q_{c},P_{c}Q_{a},P_{a}Q_{b}$ are of equal length and each of them is antiparallel to the triangle side opposite of it. Furthermore the length of the line segments equals the radius of the Cosine circle, that is:

r_{C}=|P_{b}Q_{c}|=|P_{c}Q_{a}|=|P_{a}Q_{b}|

The two triangles $\triangle P_{a}P_{b}P_{c}$ und $\triangle Q_{a}Q_{a}Q_{c}$ consisting of the six intersection points are congruent and similar to the reference triangle $\triangle ABC$ .

References[edit]

Roger A. Johnson: Advanced Euclidean Geometry. Dover 2007, ISBN 978-0-486-46237-0, pp. 273–274, 278
G. Wotherspoon: The Radii of the Cosine and Lemoine Circles In: The Mathematical Gazette, Volume 14, No. 199 (March, 1929), pp. 362-364 (JSTOR)
A. Emmerich: Die Brocardschen Gebilde und ihre Beziehungen zu den verwandten merkwürdigen Punkten und Kreisen des Dreiecks. Verlag Georg Reimer, Berlin 1891, pp. 42–52 (German)
Ross Honsberger: Episodes in Nineteenth and Twentieth Century Euclidean Geometry. MAA, 1995, pp. 87–98 (Digitalisat)

External links[edit]

Weisstein, Eric W. "First Lemoine Circle". MathWorld.
Wolfgang Ströher: Dreiecksgeometrie, lecture notes, TU Wien, pp. 90–93 (German)

Kategorie:Kreis Kategorie:Dreiecksgeometrie

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