A-Level经济学中，关于demand和supply，也就是需求和供给，有如下的知识点总结。
In a free market, the term market equilibrium indicates a situation of balance between the market forces of supply and demand where there is no tendency for change to occur under present conditions. In this situation, plans of consumers match the plans of suppliers. Any particular market equilibrium has an equilibrium price and an equilibrium quantity. At this equilibrium, total consumer expenditure and total industry revenue are equal.
If suppliers decide that consumers are willing to pay a higher price than at present they will raise their prices and income output. Then consumers buy less, the suppliers wil find that their sales fall as price in higher than the equilibrium. As a result, suppliers will soon find they have unsold goods or excess supply. This is known as a buyer's market.
On the other hand, when price is below the market equilibrium, demand exceeds supply, then there are unsafied demands exits which is called shortage, and it is a seller's market.
在自由市场中，商品的价格最终是由需求和供给的力量形成的。市场均衡就是指供给和需求处于平衡，并且两者都没有变化的倾向时的情形。这个时候，需求的量恰好等于供给的量。
当市场价格高于均衡价格时，生产者会生产较多的商品而同时消费者的需求较少。这个时候供大于求，产生剩余。这种情形下的市场叫做买方市场。此时生产者会开始降低商品售价，同时降低产量。随着价格的降低消费者的需求量开始上涨，最终达到均衡的价格和数量。
相反，当价格低于均衡价格时，求大于供，市场中存在短缺，这个情形下的市场就是卖方市场。
需要注意的是，市场从不均衡的状态达到均衡，是需要一定时间的。
以上，就是A-Level经济学中关于需求和供给的相关知识，在A-Level备考时记忆并理解这些知识点，考试时才能将知识点运用到考试题目当中去。
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