smallest isosoleez triangel=equalateral triangel (all sides the same)
Then yu wanna get the serkel INSIDE the triangel.
For equilateral triangel (oenlee equilateral) outside serkel radius=2*inside radius
outside radius=triangel side length*tan(beta)/2,
so inside radius=half this=side*tan/4
beta=30 deg for equilateral triangel, tan(30)=0.577350269
yu sae inside radius=5, so side=4*5/tan(30)
=20/0.57735
side=34.641