Q.Explain why economists say a monopolist has "no supply curve," unlike a firm under perfect competition.

Under perfect competition each price maps to one unique quantity via the MC curve; under monopoly the same price can correspond to different quantities depending on the demand curve faced.

Under perfect competition, the equilibrium rule is simply $P=MC$. This means that for ANY given price, there is exactly ONE corresponding quantity — read straight off the firm's (rising) MC curve. Because this price-to-quantity mapping holds regardless of what the demand curve elsewhere looks like, the MC curve itself can be treated as the firm's supply curve — a stable rule linking price to quantity supplied.

Under monopoly, the equilibrium rule is different: the monopolist sets output where $MR=MC$, NOT where $P=MC$. Price is then determined SEPARATELY, by reading it off the demand (AR) curve at that chosen output. Crucially, MR is derived from the ENTIRE shape of the demand curve, not just its value at one point — so a change in the SHAPE of the demand curve (say, it becomes more or less elastic, even while passing through the same current price-quantity point) will change the equilibrium quantity, WITHOUT the price him necessarily changing, or vice versa. …

Some students think a monopolist "has no supply curve" simply because it is a single firm — the real reason is technical: it is because MC is equated to MR (a demand-shape-dependent magnitude), no …