Two small beads having charges $q_{1}$ and $q_{2}$ of the same sign are fixed at the opposite ends of a horizontal insulating rod of length $d$. The bead with charge $q_{1}$ is at the origin. As shown in Figure $\mathrm{P} 22.8,$ a third small, charged bead is free to slide on the rod. (a) At what position $x$ is the third bead in equilibrium?
(b) Can the equilibrium be stable?