The intensity of the field in the dielectric medium inside a capacitor, E=σ0k∈0
Let the intensity of the field created by free charges is E0=σ0∈0 and that by the polarized charges be E′=σ′∈0. The resultant intensity E=E0−E′
σ0k∈0=σ0∈0−σ′∈0⟹σ′=σ0(1−1k)=σ0(k−1k)