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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.
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\theta
, the inscribed angle theorem should help.