Key similarities are that both the monopolistically competitive firm and the perfectly competitive firm face competition from many few other firms, produce at the point where MR > MC MR < MC MR = MC , and have a price less than equal to larger than average total cost and hence earn positive zero negative economic profits in a long-run equilibrium. Part 3 The key differences are that the demand curve for the monopolistically competitive firm slopes downward because of product homogeneity differentiation , and that its price equals exceeds is less than MR but is less than but equals and exceeds the minimum feasible ATC