When x2 >; At x 1, f(x2)-f(x 1)>0 or f (x 1)-f (x2) < 0 is increasing function, and f (x2)-f (x 1) < 0 or f (x/. 0 is the subtraction function.

That is, when the function value of the larger independent variable is greater than that of the smaller independent variable, it is increasing function; Conversely, when the function value of the larger independent variable is smaller than that of the smaller independent variable, it is a subtraction function.

That is, increasing function means that the greater the independent variable X, the greater the function value Y, and the smaller the independent variable X, the greater the function value Y. 。

Make a clear definition and see the requirements of the topic. Whether f(x2)-f(x 1) or f(x 1)-f(x2), monotonicity can be judged according to the above concepts.

I'm a senior three. You can ask me anything else I don't know.