A Geometric Mean Sangaku

It is often intimated that some of the sangaku - geometric problems carved on colorful wooden tablets - have been posted by clever teenagers. Of course, 200-400 years later, no one can be certain about that, but if there was indeed a problem suitable for an early age, the one below fits the bill (Kiyomizu Temple, Kyoto Prefecture).

 

Squares and circles are inscribed in successive right triangles. What is the relation between the radii of the circles?

Solution

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Copyright © 1996-2017 Alexander Bogomolny

 

When a square is inscribed in a right triangle with one side on the hypotenuse of the latter, it cuts off three smaller triangles all similar to the initial one. In the problem, the process is repeated with the largest of the three and then again with the largest of the newly created pieces. In all cases, a square is inscribed into similar triangles.

So again, let there be a right triangle with hypotenuse c1 and c2 be the hypotenuse of the largest of the three pieces cut off from the triangle by the inscribed square. q = c1 / c2. If we repeat the operation with the largest triangle we'll get a smaller triangle of hypotenuse c3 such that q = c2 / c3, because of the similarity.

c2 appears to be the mean proportional between c1 and c3. But then again, because of the similarity, the same is true for any linear element in the triangles, not only the hypotenuse. In particular, the radii of the three circles constructed similarly in the similar triangles satisfy the mean proportional condition:

  r1 : r2 = r2 : r3.

In other words, the radius of middle (blue) circle is the geometric mean of the radii of the two red circles.

References

  1. H. Fukagawa, D. Pedoe, Japanese Temple Geometry Problems, The Charles Babbage Research Center, Winnipeg, 1989

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    Charles Babbage Research Center
    P.O. Box 272, St. Norbert Postal Station
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Sangaku

  1. Sangaku: Reflections on the Phenomenon
  2. Critique of My View and a Response
  3. 1 + 27 = 12 + 16 Sangaku
  4. 3-4-5 Triangle by a Kid
  5. 7 = 2 + 5 Sangaku
  6. A 49th Degree Challenge
  7. A Geometric Mean Sangaku
  8. A Hard but Important Sangaku
  9. A Restored Sangaku Problem
  10. A Sangaku: Two Unrelated Circles
  11. A Sangaku by a Teen
  12. A Sangaku Follow-Up on an Archimedes' Lemma
  13. A Sangaku with an Egyptian Attachment
  14. A Sangaku with Many Circles and Some
  15. A Sushi Morsel
  16. An Old Japanese Theorem
  17. Archimedes Twins in the Edo Period
  18. Arithmetic Mean Sangaku
  19. Bottema Shatters Japan's Seclusion
  20. Chain of Circles on a Chord
  21. Circles and Semicircles in Rectangle
  22. Circles in a Circular Segment
  23. Circles Lined on the Legs of a Right Triangle
  24. Equal Incircles Theorem
  25. Equilateral Triangle, Straight Line and Tangent Circles
  26. Equilateral Triangles and Incircles in a Square
  27. Five Incircles in a Square
  28. Four Hinged Squares
  29. Four Incircles in Equilateral Triangle
  30. Gion Shrine Problem
  31. Harmonic Mean Sangaku
  32. Heron's Problem
  33. In the Wasan Spirit
  34. Incenters in Cyclic Quadrilateral
  35. Japanese Art and Mathematics
  36. Malfatti's Problem
  37. Maximal Properties of the Pythagorean Relation
  38. Neuberg Sangaku
  39. Out of Pentagon Sangaku
  40. Peacock Tail Sangaku
  41. Pentagon Proportions Sangaku
  42. Proportions in Square
  43. Pythagoras and Vecten Break Japan's Isolation
  44. Radius of a Circle by Paper Folding
  45. Review of Sacred Mathematics
  46. Sangaku à la V. Thebault
  47. Sangaku and The Egyptian Triangle
  48. Sangaku in a Square
  49. Sangaku Iterations, Is it Wasan?
  50. Sangaku with 8 Circles
  51. Sangaku with Angle between a Tangent and a Chord
  52. Sangaku with Quadratic Optimization
  53. Sangaku with Three Mixtilinear Circles
  54. Sangaku with Versines
  55. Sangakus with a Mixtilinear Circle
  56. Sequences of Touching Circles
  57. Square and Circle in a Gothic Cupola
  58. Steiner's Sangaku
  59. Tangent Circles and an Isosceles Triangle
  60. The Squinting Eyes Theorem
  61. Three Incircles In a Right Triangle
  62. Three Squares and Two Ellipses
  63. Three Tangent Circles Sangaku
  64. Triangles, Squares and Areas from Temple Geometry
  65. Two Arbelos, Two Chains
  66. Two Circles in an Angle
  67. Two Sangaku with Equal Incircles
  68. Another Sangaku in Square
  69. Sangaku via Peru
  70. FJG Capitan's Sangaku

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Copyright © 1996-2017 Alexander Bogomolny

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