A-level economics revision guides and question banks covering labour markets, supply and demand, market structure and all core economics a-level topics.
以下内容我们会持续更新：
今天带给大家Alevel经济笔记是：
成本和收入-“边际收益递减规律”The Law of Diminishing Marginal Returns
Although this topic is called 'Costs and revenues', it is important that we look at the law of diminishing marginal returns first because it is from this law that the cost curves are derived. To start with, we need to define a few terms.
尽管这个话题被称为“成本和收入”，但重要的是我们首先要研究边际收益递减规律，因为正是从这个规律得出了成本曲线。首先，我们需要定义一些术语。
总产品、平均产品和边际产品
Before commencing the bulk of the topic, it is important to make a few assumptions. The following analysis applies to the short run only.
The short run is defined as the period of time where at least one factor of production is fixed.
Only in the long run can we assume that the amount of capital (e.g. machines) can vary.
在开始本主题的大部分内容之前，重要的是要做出一些假设。以下分析仅适用于短期。短期定义为至少一个生产要素固定的时期。只有从长远来看，我们才能假设资本（例如机器）的数量会发生变化。
