Charges ±q are placed on the plates of a flat capacitor. The gap between
the plates is filled with a substance whose dielectric constant changes in the
direction perpendicular to the plates according to the law ε = ε0 (1 + x)−1,
where x is the distance to the positive plate, and d is the distance between the
plates. Find the volume charge density as a function of x. Area of plates S.