\(2{\pi}r=C\)
That is the formula for circumference.
Now to find how far off, we find the difference between the two radii when the concentric circles are formed.
We make a system of equations
\(2{\pi}x=C\)
\(2{\pi}y=(C+1)\)
We have to evaluate \(|y-x|\)
Substituting: \(2{\pi}y-1=2{\pi}x\)
Simplifying: \(1=2{\pi}y-2{\pi}x\)
Solving: \(\frac{1}{2{\pi}}=y-x\)
Solving for \(|y-x|\),
\(\boxed{\frac{1}{2{\pi}}}\)
That was my attempt 
+2864 
