Because LAC is the envelope of SAC, the scale income changes and remains unchanged for a long time.

1, the scale gain is constant: LAC=LMC, which is the straight line of LAC and the lowest point of SAC.

2. When the scale gains increase or decrease: LMC and LAC meet the lowest point of LAC, that is, the lowest point of SA corresponding to the output. The points on the left and right sides of this intersection are not the lowest SAC points of the corresponding output. On the left is the point where SAC falls, namely SAC & gtSMC, and on the right is the point where SAC rises, namely SAC.

1, display preference theory.

The theory of display preference was first put forward by Samuelson, then supplemented by Houthakker and Richter, and gradually became a system. Its appearance stems from the imperceptible utility of traditional demand theory. In the traditional micro-demand theory, the choice behavior of maximizing consumer utility is easy to analyze only when the consumer utility function is known and has good properties. But this is not the case in real life, because the utility or preference can not be directly observed, but only the consumer's choice behavior can be directly observed. If we can find some relationship between choice behavior and preference, and then if consumers' "choice" can show "preference", then demand theory and preference theory can be based on observable consumer behavior, which makes it possible to test the consistency between consumer behavior and maximization axiom. This is the basic idea of display preference theory.

2. Choice under risk conditions.

In the market with a lot of risks, how to effectively choose the combination of asset symptoms to avoid risks becomes very important. Therefore, the research on insurance market, securities market and futures contract has become a very active branch of microeconomic theory. Especially since 1960s and 1970s, with the development of cognitive psychology and other branches of psychology, people began to test the rational hypothesis and expected utility theory of classical economics. The results show that the rational axiom hypothesis is established under certain conditions, and people's behavior often violates the public rational hypothesis under vague or uncertain conditions. Therefore, when making decisions under uncertain conditions, we must examine people's complex mentality. In this case, there are wait-and-see theory, regret theory and fuzzy model.