Demand Theorem: When other factors remain unchanged, the demand of a commodity is inversely proportional to its price (negative correlation).

Supply theorem: under the condition that other factors remain unchanged, the supply and price of goods change in the same direction.

Theorem of supply and demand: under the condition that other conditions remain unchanged, the change of demand causes the change of equilibrium price and equilibrium quantity in the same direction respectively; The change of supply causes the opposite change of equilibrium price and the same change of equilibrium quantity.

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