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 Craige, The relationship involving the arc length a the radius of the circle r and the angle θ measured in radians is 
 I let the inside radius be r, the length of the inside arc be a1 and the length of the outside arc be a2. Then for the inside arc 
 and for the outside arc 
 Since the angle θ is the same for both arcs 
 Solving for r gives 
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