In mathematics, in order to use, describe and calculate more conveniently, some letters are used to represent some numbers. In mathematics, the five letters N, Z, Q, R and C also participate in the representation of some numbers. N is used to represent natural numbers, Z is used to represent integers, Q is used to represent rational numbers, R is used to represent real number sets, and C is used to represent complex number sets. If letters are used instead, workload and working hours will be greatly reduced. In mathematics, it is common to use not only these letters but also other letters instead of other letters. In some cases, we must also remember the contents of some letter substitutions.