Dlanod is building huts (the end)

Geometry Level 4

Dlanod the greedy developer is still dreaming of buying a square plot of land and building some vacation huts on in, circular as viewed from above, each with a diameter of six meters. He has not given up on his plan to build them wall-to-wall, although this might be a bit inconvenient for the unfortunate tourists who will have to stay there. Dlanod's chief architect, who went to Yale, advises him that, regardless of the details of the arrangement, he will have to buy at least 10 π 10\pi square meters of land for each hut. Dlanod states that "his intuition" tells him otherwise, and he will be able to do better. Who is right?

Prove the architect's claim, or provide a detailed plan for an arrangement (including the number of huts and the size of the plot) that proves the architect wrong.

Dlanod is right The architect is right Not enough information

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2 solutions

Otto Bretscher
Dec 24, 2018

The plan is to place the huts in a "honeycomb pattern," in p p rows with q q cottages each, as shown in the attached figure, where p = 6 p=6 and q = 7 q=7 . We can place the huts onto a rectangular plot with Length L = 6 q + 3 L=6q+3 and width W = 6 + 3 3 ( p 1 ) W=6+3\sqrt{3}(p-1) . For our construction, we will choose the largest p p such that W < L W<L , namely, p = 2 q 1 3 p=\lceil {\frac{2q-1}{\sqrt 3}}\rceil , so that the huts will fit on a square plot with side length L = 6 q + 3 L=6q+3 , with a little extra yard space left for some "luxury" huts in the top and/or bottom rows. The amount of land we need per cottage is ( 6 q + 3 ) 2 p q = ( 6 q + 3 ) 2 q 2 q 1 3 \frac{(6q+3)^2}{pq}=\frac{(6q+3)^2}{q\lceil {\frac{2q-1}{\sqrt 3}\rceil}} The limit of this expression, as we let q q go to infinity, is 18 3 < 31.2 < 10 π 18\sqrt 3<31.2<10\pi , so that the amount of land we need per cottage will be < 10 π <10\pi for sufficiently large q q , namely, for all q 200 q\geq 200 and for some smaller ones.

Once again, Dlanod is right \boxed{\text{Dlanod is right}} . The resort will be huge, of truly Dlanodian proportions.

Regardless of the details of the arrangement, Dlanod will need to buy at least 18 3 > 31.17 18 \sqrt 3 > 31.17 square meters per hut, though.

Mark Hennings
Dec 26, 2018

Dlanod's architect is saying that the density of all circle packings in a square have density that never exceeds 90 90 %.

Our friendly website tells us that the first circle packing with a density of greater than 90 90 % involves 8281 8281 circles, with the next involving 9503 9503 . As might be expected, this packing is almost hexagonal.

Our "friendly website" does not quite tell us that the first circle packing with a density of greater than 90% involves 8281 circles; it's just the first one they found. They do not claim that their values are optimal, except for the first few n n .

The case of 8281 huts is a good example, though, as we can work it out ourselves. If we use a perfectly hexagonal packing, arranging the huts in a 85 × 98 85 \times 98 pattern, with the columns alternating between 84 and 85, the density does indeed come out just a tad above 90%.

I'm puzzled by the fact that my formula also allows a density over 90% in the case of 8976 = 88 × 102 8976=88\times 102 huts, while the "friendly website" lists less than 80%. Either I made a computational error somewhere (as I often do) or their value is really bad.

Otto Bretscher - 2 years, 5 months ago

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