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Topic: how many coins?
Replies: 5   Last Post: Apr 8, 1995 3:01 PM

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Tad Watanabe

Posts: 442
Registered: 12/6/04
Re: how many coins?
Posted: Apr 8, 1995 3:01 PM
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Bravo!!

On Fri, 7 Apr 1995, Ted Alper wrote:

> Hey! I got it! Please bear with my ASCII diagrams.
>
> (for convenience, let's just worry about the CENTERS of the circles,
> which lie in a 1 x 999 rectangle formed by lopping margins of length 1/2
> off the top, bottom, and sides. These centers must all be at least
> 1 unit away from any other center.
>
> So, for example, in the square packing that gives 2000, the diagram is
>
> 1 2 3 4 5 6 ..996 997 998 999 1000
> *------*------*------*------*------*....*------*------*------*------*
> | |
> | |
> *------*------*------*------*------*....*------*------*------*------*
>
> And in the supposedly more efficient packing (which really is
> more efficient on the infinite plane) the boundary constraints only
> give us 1999:
>
> 1 2 3 4 5 6 ..996 997 998 999 1000
> *------*------*------*------*------*....*------*------*------*------*
> \ /
> \ /
> *------*------*------*------*----....----*------*------*------*
>
> But note there is some wiggle room in this! If the slanted sides are of
> length 1, the HEIGHT is only sqrt(3/2) (remember, these are the
> centers only -- and they form equilateral triangles of side length 1).
>
>
> That suggests the following shape, which takes advantage of the
> entire height and thus enables us to pack a few more in:
>
> I can't do it to scale, but let me label the rows and give their
> heights. Row D is on bottom, Row A is on top (so row A is height
> 1 above row D). Row B is height sqrt(3)/2 above row D and row
> C is height sqrt(3)/2 BELOW row A.
>
> So: to put it another way: Row B is 1 - sqrt(3)/2 below row A,
> Row C is sqrt(3) - 1 below row B, Row D is 1 - sqrt(3)/2 below row C.
>
> A ..4 ------6.. ..etc
> ... \ / ... ... \
> B 2.. \ / ..8.. \
> / \ \ / / \ \
> C / \ ..5.. / \ ..etc
> / \ ... ... / \ ...
> D 1-------3.. ..7-------9..
>
>
> the connected lines are of length 1, so 1,2,3 is an equilateral
> triangle, as is 4,5,6 and 7,8,9 etc. On the other hand, 2,3,4,5
> is a rhombus!
> You may verify that triangle 3,5,7 is isosceles, with the
> two identical sides of length 1. Since the altitude is
> 1 - sqrt(3)/2, we can use the pythagorean theorem to show
> that the length from 3 to 7 is
> 2 * sqrt( (1^2 - (1 - sqrt(3)/2)^2) )
> i.e. 2 * sqrt( sqrt(3) - 3/4)
> which is 1.981965
>
> So the total length from 1 to 5 is 2.981965
> But this means that we may iterate the 1,2,3,4,5,6 diagram
> 999/2.981965 times That's 335.01348
>
> which means we get 335 copies of the 6 points, plus 1 more
> (the very first point gets on in the 336ith iteration).
>
> THAT'S 2011. DONE!
>
>






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