If a series starts from the second item, the difference between each item and its previous item is equal to the same constant. This series is called arithmetic progression, and this constant is called arithmetic progression's tolerance, which is usually expressed by the letter D. In daily life, people often use arithmetic progression, for example, when the size of various products is graded, when the maximum size is not much different from the minimum size, it is often graded according to arithmetic progression.

2. Equal ratio series

If a series starts from the second term and the ratio of each term to the previous term is equal to the same constant, this series is called geometric series. This constant is called the common ratio of geometric series, which is usually expressed by the letter Q. In addition, each term is a geometric series with positive numbers, taking the same base to form a arithmetic progression; On the other hand, taking any positive number c as the cardinal number and a arithmetic progression term as the exponent, a power energy is constructed, which is a geometric series. In this sense, we say that a positive geometric series and an arithmetic series are isomorphic.

3. The meaning of set

Generally speaking, we call the object of study an element and the whole composed of some elements a set.