Explain why a monopolist maximizes its long-run profit by producing that output for which marginal revenue equals long-run marginal cost. What sense does this monopolist pricing differ from a perfect competitive market?

A monopoly in long run (as well as in the short-run) earns positive economic profit because it produces at a point where MR=MC. Price in monopoly is set above marginal cost in order to let the firm earn a positive economic profit. When the marginal revenue of selling a good is greater than the marginal cost, firms are making a profit on that good. A monopoly follows this Marginal decision rule and so it's profit maximization involves setting MR=MC. However, in perfect competition firms always maximize profit by setting price=MC. It means that price in monopoly is set above the price charged in perfect competition, while monopolist produces less amount of goods that are produced in perfect competition. This indicates a market inefficiency in the case of a monopoly.