When we observe the process of a phenomenon, we often encounter various quantities, some of which are constant in the process, which we call constants; Some quantities change in the process, that is, they can take different values, which we call variables.

Note: There is another quantity in the process. Although it changes, its change is extremely small relative to the object under study, and we regard it as a constant.

Representation of variables

If the change of a variable is continuous, then the range of change is usually represented by an interval.

On the number axis, the interval refers to all points on the line segment between two points.

Expression of inequality between named interval and marked interval of interval on number axis

Closed interval a≤x≤b

[A, B]

Open interval a < x < b

(A and B)

Semi-open interval a < x a≤x