If for the value x of any two independent variables belonging to an interval in I? 、x？ When x? & ltx？ There is always an f(x? )& ltf(x？ )。 Then let's assume that f(x) is increasing function in this interval.

On the other hand, if the value of any two independent variables belonging to an inner interval is x? 、x？ When x 1

If the function y=f(x) is a increasing function or subtraction function in a certain interval, then it is said that the function y=f(x) has (strict) monotonicity in this interval, and a certain interval is called a monotonic interval of y=f(x).

The increasing function or subtraction function in a certain interval is called monotone function.

Therefore, suppose that x 1 is greater than x2 in the definition domain, and find the relationship between f(x2-x2) and 0. Greater than 0 is monotonically increasing, less than 0 is monotonically decreasing.