Subsets, for example, if we call all electrical appliances set A (with any name) and all printers set B (with any name), then set B is a subset of set A ... that is, all electrical appliances include all printers. If it is changed to mathematical language, it means "If any element of set A is an element of set B, set A is called a subset of set B"

Let's talk about supplements. If it is the same example just now, similarly, if we want to represent all electrical appliances except printers, we can use our omnipotence to supplement it. It's simple. Supplement is a concept, but only if you have all the electrical appliances, otherwise you don't know what it is except the printer. In mathematical language, let S be a set, A be a subset of S, and the set of all elements in S that do not belong to A is called the complement of subset A in S.