Basic idea:
Definition: Utility cannot be measured specifically, but can be sorted according to preference procedures.
Three basic assumptions about preferences:
1. Completeness: Only greater than and less than and equal to.
2. Transmissibility
3. Unsaturation.
2. Indifference curves
Definition: A curve that represents that the consumption mix of two commodities is different but brings the same utility to consumers.
Features: 1. To the lower right, is a subtractive function.
2. There are countless indifference curves on the same plane, the farther away from the ground, the greater the utility.
3. Convex to the origin.
Commodity Marginal Substitution Rate MRS: The utility level remains unchanged, increasing the demand for a certain commodity, the demand for another commodity that needs to be abandoned. : 
The law of diminishing marginal substitution rate: Consumers are keeping their utility constant. As the quantity of one commodity increases, the quantity of another commodity that needs to be abandoned decreases.
3. Consumption Budget Line (Budget Line):
Definition: The trajectory of the point where the budget level and commodity prices are fixed and the maximum number of two commodities that consumers can buy
The budget level is the same, and the commodity consumption mix is different: P1X1 + P2X2 = K // K is income
Budget line shift: constant slope, K change: income decreases, lower left, income increases upper right
Budget line rotation: the slope changes. K does not change, P of a certain commodity changes
4. Consumer equilibrium:
Point E is the consumer's equilibrium point. After spending the budget, it is the most effective.
Equilibrium conditions: