Monopoly and Perfect Competition

Economic welfare is the base of two parties in trade. It can be quantified as the sum of consumer surplus and producer surplus.
Consumer surplus is the amount that a buyer is willing to pay for a product minus the amount the buyer actually pays while producer surplus refers to the difference between the price that a producer is willing to accept and the actual price. As we can see from figure 1, consumer surplus is the area below the demand curve and above the market price. However, producer surplus is the area above the demand curve and below the market price. Needless to say, the sum of the two parts is called economic welfare. Economic welfare will change while the market structure changes. In this essay, I will analyze how economic welfare changes if a market structure changing from perfect competition to a monopoly. However, whether the monopolist charges a single price or a multi price will affect the outcome.
Generally speaking, perfect competition is a market structure where...