Schoch line

An arbelos is a shape bounded by three mutually-tangent semicircular arcs with collinear endpoints, with the two smaller arcs nested inside the larger one; let the endpoints of these three arcs be (in order along the line containing them) A, B, and C. Let K₁ and K₂ be two more arcs, centered at A and C, respectively, with radii AB and CB, so that these two arcs are tangent at B; let K₃ be the largest of the three arcs of the arbelos. A circle, with the center A₁, is then created tangent to the arcs K₁, K₂, and K₃. This circle is congruent with Archimedes' twin circles, making it an Archimedean circle; it is one of the Schoch circles. The Schoch line is perpendicular to the line AC and passes through the point A₁. It is also the location of the centers of infinitely many Archimedean circles, e.g. the Woo circles.[1]

If r = AB/AC, and AC = 1, then the radius of A₁ is

${\displaystyle \rho ={\frac {1}{2}}r\left(1-r\right)}$

and the center is

${\displaystyle \left({\frac {1}{2}}r\left(-1+3r-2r^{2}\right)~,~r\left(1-r\right){\sqrt {\left(1+r\right)\left(2-r\right)}}\right).}$[1]

• Okumura, Hiroshi; Watanabe, Masayuki (2004), "The Archimedean circles of Schoch and Woo" (PDF), Forum Geometricorum, 4: 27–34, MR 2057752.

• van Lamoen, Floor. "Schoch Line." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein". Retrieved 2008-04-11.