Essential University Physics: Volume 1 (3rd Edition)

We know that in equilibrium position $\frac{dU}{dr}=0$ $\implies \frac{-nb}{r^{n+1}}-\frac{-a}{r^2}=0$ This can be rearranged as: $a-\frac{nb}{r^{n-1}}=0$ $\implies \frac{nb}{r^{n-1}}=a$ $r^{n-1}=\frac{nb}{a}$ $r=\sqrt[n-1] \frac{nb}{a}$ $r = 2.82 \times 10 ^ {-10} m$