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Dudeny dissection

cut a triangle into parts

Is it possible to cut a triangle into such a number of parts so that one can put a square together?

 

net Dudeney

An affirmative answer to this question was given back in 1807. In a more general form, it sounded like this: "Any two polygons of the total area should have a common cut." This is the Boyle-Gervin theorem, proved in 1807. If we have a triangle and a square, and we know that their surfaces are the same, cutting the triangle into several polygons, we can fold the square like a mosaic.

But here is a more difficult question. Is it possible to cut so that all parts are connected in a continuous chain?

 

dissectionDudeny dissection (by the name of the author), made in the form of animation, shows us how the triangle is converted into a square, and then into a hexagon and back into a triangle (Wikipedia animation movie was used). Initially, the task of cutting a triangle was proposed by Henry Dudeny in the form of a puzzle and published in the Daily Mail newspaper (issues of February 1 and 8, 1905). Later this puzzle was included in the book "Canterbury puzzles" and to this day is included in the hundred best puzzles of "all time."
The original text is as follows:

 
 
net Dudeney

Many attempts were made to induce the Haberdasher, who was of the party, to propound a puzzle of some kind, but for a long time without success. At last, at one of the Pilgrims' stopping-places, he said that he would show them something that would "put their brains into a twist like unto a bell-rope." As a matter of fact, he was really playing off a practical joke on the company, for he was quite ignorant of any answer to the puzzle[Pg 50] that he set them. He produced a piece of cloth in the shape of a perfect equilateral triangle, as shown in the illustration, and said, "Be there any among ye full wise in the true cutting of cloth? I trow not. Every man to his trade, and the scholar may learn from the varlet and the wise man from the fool. Show me, then, if ye can, in what manner this piece of cloth may be cut into four several pieces that may be put together to make a perfect square."

Now some of the more learned of the company found a way of doing it in five pieces, but not in four. But when they pressed the Haberdasher for the correct answer he was forced to admit, after much beating about the bush, that he knew no way of doing it in any number of pieces. "By Saint Francis," saith he, "any knave can make a riddle methinks, but it is for them that may to rede it aright." For this he narrowly escaped a sound beating. But the curious point of the puzzle is that I have found that the feat may really be performed in so few as four pieces, and without turning over any piece when placing them together. The method of doing this is subtle, but I think the reader will find the problem a most interesting one.

 

Solving a math problem.
Figures show that a triangular piece of matter can be cut into 4 pieces.

[string]
1. Divide AB in half at point D
2. BC split in half to point E.

 
net Dudeney
[/row]
 
4. Build an arc from point E, with radius EB. At the intersection with line AE we get point F.
5. Divide AF in half in G.
6. Draw an arc AF with center at point G.
7. Continue the EB to the intersection with the arc at the point N.
net Dudeney
 
8. From E as from the center with a radius of EH we describe the arc HJ.
net Dudeney
 
After this operation, we managed to get the first cut of the triangle - the segment EJ.
net Dudeney
 

9. Put the segment JK equal to BE.

10. From point D, lower the perpendicular to EJ with base at L.

11. From point K, we drop the perpendicular to EJ with base at M.

net Dudeney
 
We obtain segments along which cuts should be made.
net Dudeney
 
 

 
Henry Dudeney
The author of the problem, the English mathematician Henry Ernest Dyu-Deni (1857–1930), is known as one of the eminent creators of mathematical puzzles. Speaking with this task, set in a more general form, in front of the Royal Society at Burlington House, as well as at the Royal Institute, he added another figure, in which the solution of the problem is shown in a more interesting and practical form. All parts of the model can be made of mahogany, fastened with bronze hinges, so that it was convenient to show in the audience. It is easy to see that all four parts form a kind of chain. If you twist this chain in one direction, you get a triangle, and if you twist it in the opposite direction, you get a square. Now such a division of the triangle is called “Dudeny dissection” in honor of the author or “hinged cutting”, since there should be hinges at the junction points of the polygons, which make it easy to turn the structure and go from triangle to square and back.
net Dudeney
  
 

You can make such an entertaining puzzle, which allows you to convert a regular triangle into a square by an unbroken chain of polygons, independently from paper. But in order to make it movable, it is necessary that it be not a flat, but a three-dimensional construction. Therefore, each of the polygons receives the height and is transformed into a prism.

We suggest you download the net parts in the form of the 4th prism.
For each "piece" of the puzzle, we glue together a separate prism, and then we connect the individual prisms together in the form of hinges.

 

 
 
Assembly scheme:
1.  сборка деталей задачи дьюдени
2.  сборка деталей задачи дьюдени
3.  сборка деталей задачи дьюдени
 
4.  сборка деталей задачи дьюдени
5.  сборка деталей задачи дьюдени
6.  сборка деталей задачи дьюдени
 
7.  сборка деталей задачи дьюдени
8.  сборка деталей задачи дьюдени
9.  сборка деталей задачи дьюдени
 
10.сборка деталей задачи дьюдени
11.сборка деталей задачи дьюдени
12.сборка деталей задачи дьюдени
 
 
In addition to the "classic" cutting of the triangle, you can also make other, very attractive articulated cuttings of geometric shapes. 1. Articulated cutting of a square, which allows to convert it into a regular hexagon.
The task is as follows: how to cut a square so that a regular hexagon can be assembled from its parts?
шарнирное разрезание квадрата
шарнирное разрезание квадрата
шарнирное разрезание шестиугольника
 
3. Articulated cutting of the cross, formed by five squares, and allowing to convert it into a single square.
The task is as follows: how to cut a cross formed by five squares so that a single square can be assembled from its parts?
шарнирное разрезание креста
шарнирное разрезание креста
шарнирное разрезание креста
 
 
 
 
Canadian designer Emmanuel Peluchon, repeated the puzzle Dyudeni making it out of wood and placing inside the clock:
шарнирное разрезание дьюдени коллекционные часышарнирное разрезание дьюдени коллекционные часышарнирное разрезание дьюдени коллекционные часы
The design is made of wooden prisms, neatly fastened with conventional hinges.
The cost of such a designer puzzle is 380 Canadian dollars.
 
 
шарнирное разрезание модели
The picture shows all the puzzles illustrating the hinge cut, collected from the proposed reamers.
 

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