R is a set of real numbers, including rational numbers and irrational numbers, and all real numbers can be expressed on the number axis; The range of real number set is very wide. In the mathematics study before high school, all the numbers we came into contact with were real numbers.

Real numbers correspond to imaginary numbers, which cannot be expressed on the number axis. Imaginary numbers are the learning category of senior high school mathematics. Each number set is its own representation, for example, R stands for real number set, Z stands for integer set and Q stands for rational number set. The figures are divided into several groups for your understanding.

R is close to the four operations of addition, subtraction, multiplication and division (divisor is not zero). That is, the sum, difference, product and quotient (not zero) of any two real numbers are still real numbers. The set of real numbers is ordered, that is, any two real numbers A and B must satisfy one of the following three relations: A: B > C, then a> C.

Real numbers are Archimedes, that is, for any a, b- R, if b >；; A>0 has a positive integer n, which makes na >；; B the set r of real numbers is dense, that is, there is another real number between two unequal real numbers, both rational and irrational.