A-level economics revision guides and question banks covering labour markets, supply and demand, market structure and all core economics a-level topics.

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今天带给大家Alevel经济笔记是：

第三章：Elasticities—Price Elasticity of Demand

A definition and the formula

定义和公式

All elasticities measure responsiveness.

所有弹性都衡量响应性。

In this case, the two key words are 'price' and 'demand', so the price elasticity of demand measures the responsiveness of the quantity demanded to a given price change.

In the last 'topic' we discussed demand at some length. In most cases, the demand for a good rises when the price falls, ceteris paribus. The question is, by how much?

在这种情况下，两个关键词是“价格”和“需求”，因此需求的价格弹性衡量了需求量对给定价格变化的反应程度。在上一个“主题”中，我们详细讨论了需求。在大多数情况下，在其他条件不变的情况下，当价格下跌时，对商品的需求就会上升。

问题是，多少？

The following formula can be used to measure exactly how responsive demand is to a given price change:

以下公式可用于准确衡量需求对给定价格变化的响应程度：

So the algebraic terms mean:

Ed = The price elasticity of demand

Δ = 'change in'

Qd = Quantity demanded

P = Price

Using the formula

使用公式

You will only face questions that specifically ask you to calculate an elasticity in multiple-choice papers.

Having said that, essay questions often appear where you need to analyse the significance of certain elasticities. Either way, it is important that you are confident in dealing with this, relatively simple, arithmetic formula.

您只会面临专门要求您在多项选择题中计算弹性的问题。话虽如此，论文问题经常出现在您需要分析某些弹性的重要性的地方。无论哪种方式，重要的是你有信心处理这个相对简单的算术公式。

Calculating the elasticity

计算弹性

Let's start with the easiest questions you might face:

让我们从您可能面临的最简单的问题开始：

The price of a CrispyChoc Bar in the local newsagent rises from 25p to 30p. As a result, the newsagent finds that the demand for this product falls from 80 bars a day to 40 bars a day. Find the price elasticity of demand.当地报刊亭的 CrispyChoc Bar 价格从 25 便士涨到 30 便士。结果，报刊经销商发现对该产品的需求从每天 80 条下降到每天 40 条。求需求的价格弹性。

At this point, it might be worth reviewing how to calculate percentage changes.

Basically, you work out the change, divide this change by the original figure and then multiply the result by 100. Or, if you prefer the algebraic form:

此时，可能值得回顾一下如何计算百分比变化。基本上，您计算出变化，将此变化除以原始数字，然后将结果乘以 100。或者，如果您更喜欢代数形式：

So, the percentage change in quantity demanded is -40 (the change, or fall in demand) divided by 80 (the original amount demanded) multiplied by 100. -40 divided by 80 is -0.5. Multiply this by 100 and you get -50%.

因此，需求量变化的百分比是 -40（需求变化或下降）除以 80（原始需求量）乘以 100。-40 除以 80 是 -0.5。将此乘以 100，您将得到 -50%。

The percentage change in price is +5 (the change in price) divided by 25 (the original price) multiplied by 100. 5 divided by 25 is 0.2. Multiply by 100 and you get 20%.

价格变动百分比是+5（价格变动）除以25（原价）再乘以100。5除以25等于0.2。乘以 100，你得到 20%。

Now we can use the formula for the price elasticity of demand:

现在我们可以使用需求价格弹性公式：

Notice that the answer is negative. This is because the price rose (positive) causing the quantity demanded to fall (negative).

A negative divided by a positive is always negative. This is to be expected. The demand curve is nearly always downward sloping showing a negative relationship between price and quantity demanded.

Because nearly all elasticities of demand are negative examiners often don't use the negative sign.

The question will just state an elasticity of, say, 3. What they mean is -3, so don't get too confused!

请注意，答案是否定的。这是因为价格上涨（正）导致需求量下降（负）。负数除以正数总是负数。这是可以预料的。需求曲线几乎总是向下倾斜，表明价格和需求量之间存在负相关关系。因为几乎所有的需求弹性都是负数，审查员通常不使用负号。这个问题只会说明弹性，比如 3。他们的意思是 -3，所以不要太困惑！

Now try a couple for yourself. The answers will appear if you click the appropriate button:

现在为自己尝试一对。如果您单击相应的按钮，答案将出现：

A greengrocer decides to cut the price of his apples from 50p per lb to 45p per lb. He finds that his daily sales rise from 40lbs a day to 45lbs a day. What is the price elasticity of demand (ceteris paribus)?一位蔬菜水果商决定将他的苹果价格从每磅 50 便士降低到每磅 45 便士。他发现他的每日销售额从每天 40 磅增加到每天 45 磅。需求的价格弹性是多少（其他条件不变）？

The price of a litre of unleaded petrol rises from 80p to 84p (not again!). As a result, the quantity demanded at a local forecourt falls from 4000 to 3880 litres a day. What is the price elasticity of demand (ceteris paribus)?一升无铅汽油的价格从 80 便士涨到 84 便士（不会再涨了！）。结果，当地前院的需求量从每天 4000 升下降到 3880 升。需求的价格弹性是多少（其他条件不变）？

Calculating demand and price changes from a given elasticity

根据给定的弹性计算需求和价格变化

Often, you will face the following type of question:

通常，您会面临以下类型的问题：

A greengrocer decides to cut the price of his bananas from 40p per lb to 32p per lb. The price elasticity of demand for this product is 2. He currently sells 80lbs of bananas a day. How many will he sell after the price cut?一位蔬菜水果商决定将他的香蕉价格从每磅 40 便士降低到每磅 32 便士。该产品的需求价格弹性为 2。他目前每天销售 80 磅香蕉。降价后他能卖多少？

The maths doesn't really get any harder than this in A level economics. That's meant to be a good thing, by the way!

在 A-level 经济学中，数学真的没有比这更难的了。顺便说一句，这本来是一件好事！

The formula has three parts: Ed, %Qd and %P. In the questions where you had to find the value of the elasticity, you were given two of the three parts and asked to find the third. In the question above, exactly the same thing has happened except the part you need to find has changed. Always write down the formula before you do your working:

该公式包含三个部分：E d、%Q d和 %P。在你必须找到弹性值的问题中，你得到了三个部分中的两个，并要求你找到第三个。在上面的问题中，除了您需要查找的部分发生了变化之外，发生了完全相同的事情。 在你开始工作之前，一定要写下公式：

Remember that although the elasticity is stated as '2', it is, in fact, -2:

请记住，虽然弹性被表述为“2”，但实际上是 -2：

Now we can rearrange the formula (remember that GCSE maths!):现在我们可以重新排列公式（记住 GCSE 数学！）：

(a minus times a minus is a plus).（a 减乘以减号是加号）。

So the demand for the greengrocer's bananas will rise by 40%. Initial sales were 80lbs a day, so sales after the price cut will be 112lbs a day (40% of 80 is 32. Add this to 80 to give 112.)因此，对蔬菜水果商香蕉的需求将增加 40%。最初的销售额是每天 80 磅，因此降价后的销售额将是每天 112 磅（80 的 40% 是 32。将此添加到 80 得到 112。）

Now try a couple for yourself. Think carefully about the second question. The answers will appear if you click the appropriate button:

现在为自己尝试一对。仔细思考第二个问题。如果您单击相应的按钮，答案将出现：

A publican decides to increase the price of Brand X lager from £2 a pint to £2.10 a pint. The price elasticity of demand for Brand X is 0.8. He currently sells 300 pints a day. What will the new demand be (ceteris paribus)?一位老板决定将 Brand X lager 的价格从每品脱 2 英镑提高到每品脱 2.10 英镑。品牌 X 的需求价格弹性为 0.8。他目前每天卖出 300 品脱。新的需求是什么（其他条件不变）？

The same publican sells best bitter for £1.50 a pint. He finds that, as a result of a price change, he is selling 242 pints of bitter compared with 220 pints before the price change. The price elasticity of demand for best bitter is 1.25. What is the new price (ceteris paribus)?

同一位店员以每品脱 1.50 英镑的价格出售最好的苦酒。他发现，由于价格变化，他卖出了 242 品脱苦味，而价格变化前是 220 品脱。最佳苦味的需求价格弹性为 1.25。新价格是多少（其他条件不变）？

今天的Alevel经济笔记就更新到这里，如果大家对Alevel经济学感兴趣可以关注我们，

关于Alevel经济笔记第三章：Elasticities后续的更新计划如下：

之前更新：

Alevel经济学习笔记：总需求和总供给篇-Supply Side Policies Alevel经济学习A*笔记：成本和收入之成本及其曲线的工作示例 Alevel经济学习A*笔记：成本和收入之“边际收益递减规律” Alevel经济学习A*笔记：成本和收入之收入及其曲线 Alevel经济学习A*笔记：成本和收入之“利润”