If one had considered a solid cylinder of radius b, one can suppose that it is made of two concentric cylinders of radius a and the outer part, joined along the length concentrically one inside the other.If Ia and Ix are the currents flowing through the inner and outer cylinders Itotal = Ib = Ia + Ix⇒ VRb = V/Ra + V/Rxwhere Rb is the total resistance and Rx is the resistance of the tubular part.∴But∴∴