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I've been thinking about the following problem:

We have a $1\times 5$ rectangle: how to cut it and reassemble it such that it forms a square?

Thanks a lot!

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    The inverse of this question can be found at http://mathoverflow.net/questions/15181/divide-a-square-into-5-equal-squares/15183#15183.2012-01-10

5 Answers 5

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Cut a small square out of it and throw the rest away. If you are not allowed to throw any part away, then crumple the rest into a ball and stack it on the square. If you are not allowed to throw any part away and you are not allowed to overlap pieces, then I don't know the answer.

Edit:

Now there's a standard way to cut two squares into a totasl of four peices that rearrange to form a single square. I wish I knew how to draw pictures.

squares
(source: cut-the-knot.org)

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    When you copy something from another post, common practice is to acknowledge the source. But thanks for supplying the picture.2012-01-05
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enter image description here

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    The area of your original rectangle is 5. A square with the same area must have sides of length $\sqrt{5}$. So thinking of the pythagorean theorem, we need $a^2+b^2=5$. The only positive integer solution is $a,b=1,2$. This led me to chop off a rectangle of size $1\times 2$ and slice it diagonally. It then made sense to do it again and then sliding things around we get the above answer. That's how I thought about it anyway. The hard part was drawing it in Microsoft Paint :)2012-01-06
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You can do it with four pieces, and translations only (no rotations).

enter image description here

enter image description here

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    Very optimal. +1!2012-01-06
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A standard approach to finding solutions to problems like this is to overlay two tilings. In the following image, the bright yellow rectangles give one 5-piece and two 4-piece solutions requiring only translations (one of which is the same as Robert Israel’s above):

$\hspace{1.15in}$ tilings

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Here's a method that is surely far from optimal.

First, cut it into 5 $1\times1$ squares, and arrange them into a $2\times2$ square sitting next to a $1\times1$. Now there's a standard way to cut two squares into a total of four pieces that rearrange to form a single square. I wish I knew how to draw pictures. Anyway, let the small square be $ABCD$ With $C$ a vertex of the big square and $CD$ along the side $CEFG$ of the big square. Find $H$ on $CG$ such that $GH=AB$. Cut along $FH$ and along $AH$. Then the bits $FHG$, $ABH$, and $ADEFHA$ can be moved to form a square.

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    @Gerry Myerson : You have no idea what can come out of a napkin... =P2012-01-06