众所周知,若A≥B与B≥A同时成立,则有A=B.利用此“两边夹”法则,可巧妙地解决一些竞赛题. 例1设f(x)是定义在R上的函数,对任意的x∈R,都有f(x+3)≤f(x)+3和f(x+2)≥f(x)+2.设g(x)=f(x)-x. (1)求证:g(x)是周期函数; As we all know, if A ≥ B and B ≥ A are established at the same time, then there is A = B. Using this “two-side folder” rule, we can skillfully solve some contest problems. Example 1 Let f(x) be a function defined on R. For any x∈R, there are f(x+3)≤f(x)+3 and f(x+2)≥f(x)+2. Let g(x)=f(x)-x. (1) Proof: g(x) is a periodic function;