Sangaku Problems: Japanese geometrical problems or theorems, in Euclidean geometry http://en.wikipedia.org/wiki/Sangaku https://math.osu.edu/sites/math.osu.edu/files/Sangaku.pdf Two tangents to circle "c" at points B and C meet at point D. The circle "c" passes through the incenter of the triangle BCD. Incenter is a point of concurrency formed by intersection of triangle's angle bisectors.