Short-run choice follows the monopoly rule

Differentiation gives each firm a downward-sloping demand curve.

The firm chooses quantity where MR equals MC and charges the price on demand. It can earn profit, break even, or incur loss. Advertising can shift demand right, make it less elastic by building loyalty, or simply raise cost. Its profitability depends on added revenue versus added cost.

In the short run, the graph is the same decision architecture as monopoly because the differentiated firm faces downward-sloping demand. MR lies below demand. Locate q at MR=MC, move up to firm demand for price, and compare price with ATC. The label “many firms” does not make the individual seller a price taker.

The firm may earn profit if price exceeds ATC, incur loss if price is below ATC but covers AVC, or shut down if price is below AVC. Entry has not yet had time to change the number of close substitutes. Short-run profit status is therefore not fixed by the market-structure label.

Product differentiation can be physical or perceived. Location and service are real differences even when the basic product is similar. Because close substitutes exist, demand is usually more elastic than monopoly demand but not perfectly elastic like a competitive firm’s.

Differentiation can involve features, quality, convenience, atmosphere, brand, or information. It gives the seller a limited customer base but also creates cross-price response with rivals. If one restaurant raises price, some customers remain due to location or preference while others switch.

Short-run differentiated-firm choice

A salon’s MR meets MC at 80 appointments. Its demand curve gives a price of $45, and ATC is $38. The salon earns (45-38)80=$560. It does not charge the MR value and it does not expect the profit to persist if comparable salons can enter.

Advertising should be treated as an investment. If an extra $1,000 campaign shifts demand enough to add more than $1,000 to profit, it is worthwhile. A sales increase alone does not prove success because the campaign and added service have costs. Informative advertising can increase elasticity by helping comparison, while loyalty advertising can reduce it. Use the stated mechanism.

Markup does not guarantee profit. A firm can have P>MC but P<ATC because fixed cost or weak demand produces a loss. Allocative markup and accounting of total cost are separate.

The short run assumes the number of firms fixed. If the question asks what occurs after profit attracts entry, shift to the long-run tangency analysis rather than leaving the initial demand curve unchanged.

The firm’s demand can shift without the market having a single homogeneous supply-and-demand graph. A successful advertisement, a new feature, or a rival’s exit can raise the firm’s demand and MR. An input-price change shifts MC. Recompute the intersection rather than applying a competitive price-taking response.

If advertising raises revenue by less than its cost, the campaign lowers profit even when sales rise. Strong answers compare incremental profit, not market share or total revenue alone. The same marginal logic governs the advertising decision.

Because firm demand slopes downward, P>MC can hold whether the firm earns profit or loss. Markup reflects differentiation. Profit also requires price to cover ATC. The two conclusions must be checked separately.