# GeoGebra – Malfatti’s Problem

In Axiomatic Geometry, I and a partner explored Malfatti’s problem (see references below). I created a GeoGebra worksheet to support the presentation we later gave. I share some details here.

Malfatti originally asked this question is 1803 and the question, “How to arrange in a given triangle three non-overlapping circles of greatest area?” remained open until 1994.

As far back as 1384, in a manuscript of Italian mathematician Gilio de Cecco da Montepulciano a problem is presented of how to construct 3 circles, inside a triangle, that are tangent to each other and tangent to 2 edges of triangle. The same problem was posed by 18th century Japanese mathematician Ajima Naonubu. In 1803, Malfatti refined the problem to include minimizing the amount of area in the triangle “wasted”, i.e., constructing the largest circles possible. In 1826, Steiner (that sly old fox) developed a construction of three circles, tangent to each other and touching 2 edges of the triangle.  This was a solution to the problems posed by Montepulciano and Naonubu, but not Malfatti.  Stiener demonstrated 3 circles in a triangle but did not minimize the area “wasted”. In 1930, Lob and Richmond showed Malfatti’s circles do not produce the largest circles (i.e., minimal waste).  Optimal circles are obtained using greedy algorithm. In 1967, Goldberg showed that the Mafatti circles will never produce greatest area and the Lob and Richmond ‘greedy algorithm’ always produces the largest total area.  In 1994, Zalgaller and Los classified all the different ways 3 circles can be arranged in a triangle and developed a formula for determining which method of arranging the circles is optimal for a given triangle.  They also proved that there are 14 combinations of circles in a triangle that will yield optimal area.

Essentially, if there is room for a circle to ‘grow’ then the system is NOT rigid.  The two examples on the left are rigid, the one on the right is not, the middle circle could change radius or ‘grow’.

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Zalgaller and Los proved the result systematically, reducing all possibilities to 14 combinatorially, non-identical, rigid configurations of three circles in a triangle. See cases below.

• How can we arrange in a given triangle three non-overlapping circles of greatest area?
• For an Isosceles triangle, the first two circles are drawn as K1 and K2. Then we choose K3a or K3b depending on the relationship between sin(alpha) and tan(beta/2).
• Play around with the triangle and tell me –
• Do we use K3a or K3b when sin(alpha) > tan(beta/2)?
• Do we use K3a or K3b when sin(alpha) < tan(beta/2)? W
• hat do we do when sin(alpha) > tan(beta/2)?
• Also, are the three circles always symmetric for an Isosceles triangle?

References

Andreatta, M, Bezdek, A, & Boronski, J (2010) The Problem of Malfatti: Two Centuries of Debate. The Mathematical Intelligencer.

Coolidge, J. L. (1923). Some Unsolved Problems in Solid Geometry. The American Mathematical Monthly, 30(4), 174-180.

Goldberg, M. (1967). On the original Malfatti problem. Mathematics Magazine, 40(5), 241-247.

Zalgaller, V. A., & Los’, G. A. (1994). The Solution Of Malfatti’s Problem. Journal Of Mathematical Sciences, 72(4), 3163-3177.

Presentation Reference

Humes, W.R. & Stehr, E.M. (2013). Malfatti’s Marble Problem. Presentation prepared for final project in D.S. Durosoy’s MTH 432: Axiomatic Geometry course.

## 4 responses to “GeoGebra – Malfatti’s Problem”

1. Jan Peter says:

I am really amazed that it took so much time to find the correct result for such an elementary problem. You, too, should correct the slide no. 5: in one case K3 touches K1, in the other case K2.

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2. Thank you for your comment, Jan! Yes, it’s true that what is elementary today was not elementary 50 years or 100 years ago.

For Slide Number 5, let me see if I can understand your correction. What I show in that slide is that K3 either touches K1 or K2 (as you mention). So, I created two circles labeled 3a and 3b to illustrate three cases (left somewhat open for students to explore). In one case, 3a is larger, then we call it K3 (and thus K3 touches K1). In a second case, 3b is larger, then we call it K3 (and thus K3 touches K2). In a third case, the circles 3a and 3b are the same size, and we can arbitrarily choose one of them to be the third circle.

Does that respond to your comment? If not, please include additional detail so that I may correct my mistake. Thank you!

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3. Jan Peter says:

What I meant that you have two times the same sentence in picture 5

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• Ah, yes! I think I understand. I believe my goal was to show two of the cases, but you are right – it is unclear and seems to clutter the workspace. Thank you for the feedback!

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