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iSquared Magazine

The Daily Rant

Puzzles
Kissing Circles

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Suppose that four circular disks of equal radius are arranged as in the diagram so that they just touch without overlapping. Suppose that we can just barely fit a circle of radius 1 into the gap between these four circles. What is the radius of one of these four circles? Remember to give an exact answer -- no approximations allowed!

Courtesy of Mark Nielsen

 
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terveloc18:41:23, 17 Nov 07
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I got 1 + sqrt(2).

 
Jackson16:43:32, 09 Oct 07
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ooh, bad Lawrence, didnt turn it into simplest radical form.... tsk tsk tsk... it should be (square root 2 +1) Just like sean bow.
isnt that simpler?

 
Lawrence Lee13:17:49, 09 Oct 07
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Okay, this is pretty easy. You have a right triangle formed by the center of the inner circle, the center of any of the outer circles, and the point where two outer circles meet. Once you draw that triangle the solution becomes pretty straightforward as you realize that the hypotoneus of this triangle equals the radius of the inner circle plus the radius of the outer circle - r+1.

Therefore r+1 = square root of 2 x r and if you solve for r then r = 1 / (square root of 2 - 1)

 
Mabus17:13:57, 07 Oct 07
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Try this: (2router)^2+(2router)^2 = (2router + 2rinner)^2
Pythagorean theorem
(the centers of the outer circles form a square = 2 triangles. )

<=> result of seanbow ...
<=> router = 1/(0.414213562...)
<=> router = 2.414213565 --> compare to drawing... works

 
seanbow13:58:34, 07 Oct 07
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Or more generally:
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seanbow13:47:17, 07 Oct 07
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Hm. I got 1 + sqrt(2) for the answer.

 
Gonzo10:48:23, 07 Oct 07
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are you sure the answer cant be aproximated?

 


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