Neuberg Sangaku
Many of sangaku problems have been thought up in the West, sometimes earlier and sometimes later than they have been published in Japan. For researchers into the Japanese mathematics (wasan) of the Edo period this is of course important to identify the instances where precedence lies in the East. A construction of three tangent circles in what's now known as Malfatti's problem after an 1803 article by Gian Francesco Malfatti (17311807) has been proposed some 30 years ealier by the great Japanese mathematican Ajima Naonobu.
The applet below presents a problem posed by J. J. B. Neurberg in 1896 (Methesis, p. 193, problem 1078):
Given a ΔABC, let R, r, r_{a}, r_{b}, r_{c} be the radii of the circumcircle (O), the incircle (I) and the excircles (I_{a}), (I_{b}) and (I_{c}). The fourth tangents common to pairs of the excircles (I_{a}), (I_{b}) and (I_{c}) form a triangle A'B'C'. Show that centers of the incircle of ΔA'B'C' and the circumcircle of ΔI_{a}I_{b}I_{c} coincide and that the radius r' of the former satisfies:

The result 2r' = r + r_{a} + r_{b} + r_{c} is known to have been presented in 1803 by Yamamoto Norihisa on a sangaku at the Echigo Hukasan shrine, Niigata prefecture. The tablet has been lost but was recorded in Nakamura's 1830 manuscript Saishi Shinzan.
What if applet does not run? 
References
 Fukagawa Hidetoshi, Tony Rothman, Sacred Mathematics  Japanese Temple Geometry, Princeton University Press, 2008, pp. 298299
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 Equilateral Triangles and Incircles in a Square
 Five Incircles in a Square
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 Four Incircles in Equilateral Triangle
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 Malfatti's Problem
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 Sangaku with Quadratic Optimization
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 Sangaku with Versines
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 Sequences of Touching Circles
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 Tangent Circles and an Isosceles Triangle
 The Squinting Eyes Theorem
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 Two Arbelos, Two Chains
 Two Circles in an Angle
 Two Sangaku with Equal Incircles
 Another Sangaku in Square
 Sangaku via Peru
 FJG Capitan's Sangaku
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