In the short-run, a firm in monopolistic competition will aim to maximize its profits by producing at a level where marginal revenue (MR) equals marginal cost (MC). However, due to the presence of differentiated products, the demand curve for a firm's product will be downward sloping and more elastic than a monopolist's demand curve.

As a result, the firm will charge a price higher than its marginal cost, but lower than a monopolist's price. This is because the firm has some market power, but faces competition from similar products.

In the short-run equilibrium, the firm will earn positive economic profits if its average total cost (ATC) is lower than the price it charges. If ATC is higher than the price, the firm will incur losses.

The short-run equilibrium for a firm in monopolistic competition is shown in the diagram below:


In the diagram, the firm's demand curve (D) is downward sloping and more elastic than a monopolist's demand curve. The marginal revenue (MR) curve is also downward sloping and lies below the demand curve.

The firm will produce at the level where MR=MC, which is Q1. At this level of output, the firm will charge a price of P1, which is higher than the marginal cost (MC) of producing the product.

The firm's average total cost (ATC) curve is U-shaped, with minimum average total cost (AC) at output level Q2. If the price charged is higher than the minimum ATC, the firm will earn positive economic profits, as shown by the shaded rectangle. If the price charged is lower than the minimum ATC, the firm will incur losses.

Overall, in the short-run, a firm in monopolistic competition will aim to maximize its profits by producing at a level where MR=MC and charging a price higher than its marginal cost. However, due to the presence of competition, the firm's profits will be lower than a monopolist's profits.