It can be seen that the landlord's mathematical level is indeed … the solution is like the above two, let me tell you about the set! Collection, as its name implies, is a collection of things put together in childhood. It's like a container, and its contents are called elements. When there is nothing in the container, it is called an empty set, and if there is something, it is a non-empty set. Braces are symbols of a set, equivalent to a container. Sometimes there is this small vertical line "|" in the collection, and there will be letters or coordinate points in front of it, indicating that all elements in this collection are for it. For example, there are two oranges and three bananas in a basket. If it is written in the form of a set, the basket is braces, because there are two things in the basket. If I want to explain the number of oranges, then "|" is {y|y=3}, and the small vertical bar indicates who the elements of the collection are for. The nature of set is deterministic, mutual and disorderly, which is a little knowledge point that is often tested. In addition, to understand the relationship between elements and sets, elements are not necessarily sets, because the elements of a set can be sets, just as a big box can hold a small box. Well, I hope you have a certain understanding of the combination and know the nature of the set, then the above problem will be easy, and the solution is above, please refer to it. I hope it helps you!