Suppose the market demand curve for steel industry is given by: QT = 1000 – 3P, where QT = QF + QD and QT is the total quantity, QF is the quantity supplied by fringe firms, and QD is the quantity supplied by the dominant firm. The dominant firm’s total costs are given by TCD = 100QD and the competitive fringe’s supply curve is given by QF = -120 + P. Calculate: The dominant firm’s residual demand curve for prices between P1 and P2.

题目：Suppose the market demand curve for steel industry is given by: QT = 1000 – 3P, where QT = QF + QD and QT is the total quantity, QF is the quantity supplied by fringe firms, and QD is the quantity supplied by the dominant firm. The dominant firm’s total costs are given by TCD = 100QD and the competitive fringe’s supply curve is given by QF = -120 + P. Calculate: The dominant firm’s residual demand curve for prices between P1 and P2.