Three concentric spherical metallic shells A, B and C of radii a,b and c (a<b<c) have charge densities σ,−σ and σ respectively. If the shells A and C are at the same potential then the relation between a, b and c is :
VA=σ4πa24πϵ0a+(−σ)4πb24πϵ0b+σ4πc24πϵ0c
=σϵ0(a−b+c)
Similarly, VC=σϵ0(a2c−b2c+c)
But VA=VC
a−b+c=a2−b2c+c
Therefore, a+b=c