1. The problem statement, all variables and given/known data This is the theory for a laboratory session. If we have two hollow cylinders, one with radius r1 and the other with radius r7, the center of the first one being placed at a distance d from the center of the second one, what are the surfaces having the same electric potential? 2. Relevant equations I would like to know the meaning and the derivation of those three equations in the picture. The book says that they replace the cylinders with their electrical axis placed at the points A and B, whose positions are determined using the electrical images method (this is my translation, it might be incorrect). 3. The attempt at a solution I thought that the electric field lines must be perpendicular both to the "inside" and the "outside" cylinders, but, unfortunately, I can't go any further. Thank you!