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Previous iteration: #739914 (answer in #739973)
Please let me know if you prefer harder or easier problems. I cannot satisfy everyone with every puzzle but I can at least try to alternate difficulties.
Is there a unique solution? This is as far as I got
k = A + B 
A = 2r \sin \frac{\theta}{6}
A+2B = 2r \sin \frac{\theta}{2}
2A+2B = 2r \left( \sin\frac{\theta}{2} + \sin\frac{\theta}{6} \right)
2k = 2r \left( \sin\frac{\theta}{2} + \sin\frac{\theta}{6} \right)
\frac{k}{r} = \sin\frac{\theta}{2} + \sin\frac{\theta}{6}
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I got
\frac{r}{k} = \frac{1}{ (1+\frac{1}{\sqrt{2}})*2*sin{\frac{\theta}{6}} }
which seems to be equivalent.
\theta appears to be 3*45°, but I don't know why.
Is k=II twice I, the arc length?
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I didn't read it that way, but I was wondering if the fact that the triangle is right-isosceles imposes some kind of additional condition that lets us find \theta.
Don't really have the time to work it further right now though. Geometry isn't really my forte.
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I need to spend some time myself analyzing your answers, but for \theta, the inscribed angle theorem should help.
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Ah, yes!
The two green c have equal lengths.
Hence, the angles in the bottom right corner must be equal.
They both add up to 45°, because the triangle is right-isosceles, hence they are \frac{45°}{2}.
Now the inscribed angle theorem says \theta = 3*45°.
Therefore, \frac{r}{k} = \sqrt{2-\sqrt{2}} \approx{0.76537}
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I have \frac{\sqrt{2+\sqrt{2}}}{\sqrt{2}+1} which also \approx 0.76537 if I'm not mistaken.
Your expression is even simpler.
Good job :)
A GitHub PR for collapsable answers is on its way in next few days. We'll be able to hide spoilers which should increase the fun factor.
I don't have the bandwidth for this one today, but I do like the harder puzzles.
I'll circle back to check on other people's solutions.
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