For example, [2.5]=2, [-2.7]=-3, [4]=4 and so on.

Because m is not necessarily an integer, we use the integer function [M+ 1] to get an integer greater than m. Of course, many proofs are written as [M]+ 1, which makes it easier to understand that it is an integer greater than m, because when x = kπ (when k is an integer), cos (kπ) = 1.

So x = kπ (when k is an integer), |cosx|= 1, |xcosx|=x, then when x=[M+ 1]π, because [M+ 1] is a positive integer, | xcosx | = | [m

The main thing is to understand that [x] is called an integer function, which means the largest integer not greater than x, and the rest should be easy to understand.