In the purely competitive analysis, there were two dissimilar models, one model for the industry, in which the interaction of supply and demand recognized the market price and quantity. The second model was that of the firm, the firm faced a perfectly elastic demand curve, in which demand, price, average revenue and marginal revenue were all the similar.  Though, in the analysis of a monopoly there is but one model.  The firm, in monopoly, is the industry (by definition).  Because the firm is the industry it thus 180 faces a downward sloping demand curve, which is also the average revenue curve for the firm (as the industry).