正确答案是-8.但我怎么也弄不出来-8啊，大家能帮忙解释下吗？谢谢！

A power generation company is a monopoly that has very high barriers to entry. The quantity demand (QD) for its product is QD = 800 – 0.25 × P (where P is price). The slope of the marginal revenue curve is closest to:

Incorrect.

Solve for P from the quantity demanded:

Q = 800 - 0.25 P
P = 3200 - 4Q
TR = P × Q = 3200 Q - 4 Q2
and Marginal Revenue = ΔTR/ΔQ = 3200 - 8 Q
Therefore the slope of the curve is -8.

CFA Level I
"The Firm and Market Structures," by Richard G. Fritz and Michele Gambera
Section 6.3