# Miquel's theorem

A diagram showing circles passing through the vertices of a triangle ABC and points , and on the adjacent sides of the triangle intersecting at a common point, M.
The Pivot Theorem for various triangles

Miquel's theorem is a result in geometry, named after Auguste Miquel,[1] concerning the intersection of three circles, each drawn through one vertex of a triangle and two points on its adjacent sides. It is one of several results concerning circles in Euclidean geometry due to Miquel, whose work was published in Liouville's newly founded journal Journal de mathématiques pures et appliquées.

Formally, let ABC be a triangle, with arbitrary points , and on sides BC, AC, and AB respectively (or their extensions). Draw three circumcircles (Miquel's circles) to triangles AB´C´, A´BC´, and A´B´C. Miquel's theorem states that these circles intersect in a single point M, called the Miquel point. In addition, the three angles MA´B, MB´C and MC´A (green in the diagram) are all equal, as are the three supplementary angles MA´C, MB´A and MC´B.[2][3]

The theorem (and its corollary) follow from the properties of cyclic quadrilaterals. Let the circumcircles of A'B'C and AB'C' meet at ${\displaystyle M\neq A.}$ Then ${\displaystyle \angle A'MC'=2\pi -\angle B'MA'-\angle C'MB'=2\pi -(\pi -C)-(\pi -A)=A+C=\pi -B,}$ hence BA'MC' is cyclic as desired.

## Pivot theorem

If in the statement of Miquel's theorem the points , and form a triangle (that is, are not collinear) then the theorem was named the Pivot theorem in Forder (1960, p. 17).[4] (In the diagram these points are labeled P, Q and R.)

If , and are collinear then the Miquel point is on the circumcircle of ∆ABC and conversely, if the Miquel point is on this circumcircle, then , and are on a line.[5]

## Trilinear coordinates of the Miquel point

If the fractional distances of , and along sides BC (a), CA (b) and AB (c) are da, db and dc, respectively, the Miquel point, in trilinear coordinates (x : y : z), is given by:

${\displaystyle x=a\left(-a^{2}d_{a}d_{a}^{\,'}+b^{2}d_{a}d_{b}+c^{2}d_{a}^{\,'}d_{c}^{\,'}\right)}$
${\displaystyle y=b\left(a^{2}d_{a}^{\,'}d_{b}^{\,'}-b^{2}d_{b}d_{b}^{\,'}+c^{2}d_{b}d_{c}\right)}$
${\displaystyle z=c\left(a^{2}d_{a}d_{c}+b^{2}d_{b}^{\,'}d_{c}^{\,'}-c^{2}d_{c}d_{c}^{\,'}\right),}$

where d'a = 1 - da, etc.

In the case da = db = dc = ½ the Miquel point is the circumcentre (cos α : cos β : cos γ).

## A converse of Miquel's theorem

The theorem can be reversed to say: for three circles intersecting at M, a line can be drawn from any point A on one circle, through its intersection with another to give B (at the second intersection). B is then similarly connected, via intersection at of the second and third circles, giving point C. Points C, A and the remaining point of intersection, , will then be collinear, and triangle ABC will always pass through the circle intersections , and .

 Miquel and Steiner's Quadrilateral Theorem Miquel's Pentagon Theorem Miquel's six circles theorem states that if five circles share four triple-points of intersection then the remaining four points of intersection lie on a sixth circle

## Similar inscribed triangle

If the inscribed triangle XYZ is similar to the reference triangle ABC, then the point M of concurrence of the three circles is fixed for all such XYZ.[6]:p. 257

## Miquel and Steiner's quadrilateral theorem

The circumcircles of all four triangles of a complete quadrangle meet at a point M.[7] In the diagram above these are ∆ABF, ∆CDF, ∆ADE and ∆BCE.

This result was announced, in two lines, by Jakob Steiner in the 1827/1828 issue of Gergonne's Annales de Mathématiques,[8] but a detailed proof was given by Miquel.[7]

## Miquel's pentagon theorem

Let ABCDE be a convex pentagon. Extend all sides until they meet in five points F,G,H,I,K and draw the circumcircles of the five triangles CFD, DGE, EHA, AIB and BKC. Then the second intersection points (other than A,B,C,D,E), namely the new points M,N,P,R and Q are concyclic (lie on a circle).[9] See diagram.

The converse result is known as the Five circles theorem.

## Miquel's six circle theorem

Given points, A, B, C, and D on a circle, and circles passing through each adjacent pair of points, the alternate intersections of these four circles at W, X, Y and Z then lie on a common circle. This is known as the six circles theorem.[10] It is also known as the four circles theorem and while generally attributed to Jakob Steiner the only known published proof was given by Miquel.[11] Wells refers to this as Miquel's theorem.[12]

## Three-dimensional version of Miquel's theorem

The three-dimensional case: the four spheres intercepts other spheres on black circles.

There is also a three-dimensional analog, in which the four spheres passing through a point of a tetrahedron and points on the edges of the tetrahedron intersect in a common point.[3]

## Notes

1. ^ A high school teacher in the French countryside (Nantua) according to Ostermann & Wanner 2012, p. 94
2. ^ Miquel, Auguste (1838), "Mémoire de Géométrie", Journal de Mathématiques Pures et Appliquées, 1: 485–487
3. ^ a b Wells 1991, p. 184 - Wells refers to Miquel's theorem as the pivot theorem
4. ^ Coxeter & Greitzer 1967, p. 62
5. ^ Smart 1997, p. 177
6. ^ Francisco Javier Garc ́ıa Capita ́n, "Locus of Centroids of Similar Inscribed Triangles", Forum Geometricorum 16, 2016, 257–267.http://forumgeom.fau.edu/FG2016volume16/FG201631.pdf
7. ^ a b Ostermann & Wanner 2012, p. 96
8. ^ Steiner, J. (1827/1828), "Questions proposées. Théorème sur le quadrilatère complet", Annales de Mathématiques, 18: 302–304
9. ^ Ostermann & Wanner 2012, pp. 96–97
10. ^ Pedoe 1988, p. 424
11. ^ Ostermann & Wanner 2012, p. 352
12. ^ Wells 1991, pp. 151–2