Question: how to calculate distance between two spheres?
Answer: In a metric space (X,d) the distance between two subsets A,B of X is given as the infimum of the distance between two points, one from each of the two sets: Distance(A,B)=inf{d(a,b)∣a∈A and b∈B}. Distance ( A , B ) = inf { d ( a , b ) ∣ a ∈ A and b ∈ B } .