It is true that any one is brought into the domain (it may be a real number).

Always present:

In the definition domain, there are always numbers that make it hold. Even if there is 1, it is not necessarily a number, but it is always the case that all numbers hold.

For example: x+9

x+9 & lt； 10, in x

Therefore, in comparison, the establishment of invariance means that all the numbers to be brought must make the meaning of the question true, and there must be numbers that meet the meaning of the question, but the numbers in a given range may not all meet. .