The potential energy of a particle in a force field is: U =A/(r^2)-B/r, where A and B are positive constants and r is the distance of particle from…

The potential energy of a particle in a force field is : $\displaystyle \mathrm{U}=\frac{\mathrm{A}}{\mathrm{r}^2}-\frac{\mathrm{B}}{\mathrm{r}}$, where A and B are positive constants and r is the distance of particle from the centre of the field. For stable equilibrium, the distance of the particle is :