Finding the Monopoly Price and Quantity

A monopolist is the only seller, so its demand curve is the whole market's demand curve, downward sloping. To sell one more unit, it has to lower price on every unit sold, not just the last one. That means marginal revenue falls faster than demand and sits below it. Quantity still gets chosen the same way every firm chooses quantity: produce until the next unit's extra revenue (MR) no longer covers its extra cost (MC). Once you have that quantity, price is a separate question, answered by demand, not by MR. Demand tells you the highest price buyers will pay for that many units, so you go up from the MR = MC quantity to the demand curve, then over to price.

This two-step process, quantity from MR = MC, price from demand, is the whole mechanism. Graphically: draw demand and MR, find where MR crosses MC, drop straight down to read quantity on the horizontal axis, then go straight up from that quantity to the demand curve, then across to the price axis. The price you land on sits above marginal cost, which is how the monopolist earns profit that a competitive firm, forced to price at marginal cost, could not.

This sets up everything else in the unit. Because price sits above marginal cost, some mutually beneficial trades never happen, producing deadweight loss, the efficiency cost of monopoly power compared to competitive markets covered next.