Because the firm charges each consumer her reservation price, it is profitable to expand output to Q**.When only a single price, P*is charged,.the firm's variable profit(可变利润)is the area between the marginal revenue and marginal cost curves. With perfect price discrimination,this profit expands to the area between the demand curve and the marginal cost curve.Because the firm charges each consumer her reservation price, it is profitable to expand output to Q**. When only a single price, P* is charged, the firm’s variable profit (可变利润) is the area between the marginal revenue and marginal cost curves. With perfect price discrimination, this profit expands to the area between the demand curve and the marginal cost curve