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Why does a Perfect Competition firms demand curve is also its marginal revenue curve?


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For a perfectly competitive firm with no market control, the marginal revenue curve is a horizontal line. Because a perfectly competitive firm is a price taker and faces a horizontal demand curve, its marginal revenue curve is also horizontal and coincides with its average revenue (and demand) curve.

Yes - what you must remember is that a firm's demand curve in perfect competition is its average revenue curve. Average revenue = price x quantity / quantity = price. The demand curve shows the quantity demanded at varying prices and this is exactly what the average revenue curve will do.

Because there are so many sellers in the market, no one firm has enough market power to influence price (if a firm tried to raise price consumers would move to different suppliers; nobody would buy the good), therefore price is determined by industry supply and demand, and a firm can produce any quantity at this price . This means that the firm faces a horizontal average revenue (demand curve) and if average revenue is constant, this means total revenue is increasing at a constant rate, and therefore marginal revenue is constant as well.