Potential energy function describing the interaction between two atoms of a diatomic molecule is $U(r)=a/r^{12} - b/r^6$.Force acting between them will be zero when the distance between the would be?

Question

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$(2a/b)^{1/6} (b/2a)^{1/6} (a/b)^{1/6} (b/a)^{1/6}.$