Apply gauss's theorem to calculate the electric field due to an infinite plane sheet of charge

Let us consider a surface having charge density σ, area A, and charge Q. To calculate electric field at a distance r from the surface at a point p we consider a cylindrical Gaussian surface having length R and area of end surface A. Hence flux

         φ=EA+EA=2EA---------(1)
againφ=Q/∈₀=σA/∈₀---------(2)
from (1) & (2)
          2EA=σA/∈₀
        ⇒E=σ/2∈₀