Show that a necessary condition for a system to be in equilibrium under the conditions of the

Question:

Show that a necessary condition for a system to be in equilibrium under the conditions of the potential shown in Fig. 13.4, it must be at the position corresponding to the coordinates of point $\mathrm{A}$, or $\mathrm{B}$, or $\mathrm{C}$. Are there other equilibrium points in the region of $r$ shown in Fig?

Fig. 13.4

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