(Economics) 4個市場的Demand Curve同Marginal Revenue有著什麼關係?

For price taking market, the demand curve faced by individual seller is horizontal. So the marginal revenue curve coincides wih the demand curve. Note that no alternative pricing arrangement is practised in this market.

For monopoly(and I guess other price searching market), the demand curve faced by each seller is downward sloping as the seller has certain monopoly power to bar some competition. Under simple uniform pricing, the marginal revenue curve lies below the demand curve. If the demand curve is a straight line, marginal revenue curve bisects the demand curve. However, the relationship also depends on pricing arrangements. For example, in the case of perfect price discrimination, the marginal revenue curve will shift up and coincide with demand curve again.

Marginal Revenue 係由 Demand Curve 引申出黎
所以 D=2MR

為何會有4個市場?