328. On two coaxial infinite cylinders of radii R and 2R, charges with surface densities σ1 and σ2 are uniformly distributed (Fig. 26). It is required: 1) using the Ostrogradsky-Gauss theorem: find the dependence E (r) of the electric field strength on the distance for three regions: I, II and III. Take σ1 = -2σ, σ2 = σ; 2) calculate the strength E at a point distant from the axis of the cylinders at a distance r, and indicate the direction of the vector E. Take σ = 50 nC / m2, r = 1.5R; 3) build a graph of E (r).