1, real number set and function 1, and real numbers are closed, ordered, size-transitive, Archimedean, dense, and correspond to points on the number axis one by one.

2. Trigonometric inequality 2. The infimum principle sets s as a set of non-empty numbers. If s has an upper bound, then there must be an supremum. If s has a lower bound, then there must be a lower supremum.

3. There are three main ways to express the function concept function, namely analytical method (or formula method), list method and image method.