# Mixtilinear Circles and Concurrence

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A Mathematical Droodle

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Copyright © 1996-2018 Alexander BogomolnyL. Bankoff has introduced the term *mixtilinear* for a curvilinear triangle with two sides straight line segments and the third an arc of a circle. He also called *mixtilinear* the incircles of the mixtilinear triangles. Given a circle and a triangle (with none of the sides touching the circle), there exist circles tangent two the given circle and some two sides of the triangle. These, too, are called *mixtilinear* and, depending on the manner of the tangencies there are mixtilinear incircles and excircles.

The applet displays a circle, a triangle ABC and three mixtilinear incircles tangent to a given circle with tangency point A' opposite A, B' opposite B, and C' opposite C. We have the following result:

(1) | The three lines AA', BB', CC' are concurrent in point M say. Moreover, if I is the incenter of ΔABC and O is the center of the given circle, then points M, O, I are collinear. |

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The theorem has been discovered by Matthew Lee when yet in high school. (Lee's statement requires that the triangle be entirely inside the circle, but the proof does not use this property.) For the case where points A, B, C lie on the given circle it was also shown (even in two ways) by Paul Yiu. Lee's proof is an extension of one of Yiu's and is based on an observation that the product of two homotheties is either another homothety or a translation.

Let C(p, r) denote the circle with center p and radius r. Let

First focus on vertex A. Let C(O_{A}, R_{A}) be the circle tangent to AB, AC and

Circles C(I, r) and C(O_{A}, R_{A}) are homothetic with center A and coefficient AO_{A}/AI. _{A}/AI)_{A}, R_{A})_{A}. _{A})_{A}/AI·A'O/A'O_{A} ≠ 1.

But the same homothety can be constructed starting with vertices B and C and found to be on the lines BB' and CC', wherefrom the three lines AA', BB', CC' are indeed concurrent. Since this homothety maps

**Note**: Mixtilinear circles admit simple construction.

### References

- P. Yiu,
__Mixtilinear Circles__,*Am Math Monthly*, v. 106, n. 10 (Dec., 1999), 952-955 - M. Lee,
__Four circles and a triangle__,*The Math Gazette*, v. 90, n. 517 (Mar., 2006), 130-131

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