$$\color{rgb(0,0,255)}f(x)=\frac{x\cos x}{x^2+1}$$

$$\color{rgb(127,0,0)}u(x)=\frac{x}{x^2+1}$$

$$\color{rgb(0,127,0)}v(x)=\frac{-x}{x^2+1}$$

$$\color{rgb(127,0,127)}\begin{array}{l} \text{Pour } x \ge 0, v(x) \le f(x) \le u(x) \\ \text{Pour } x \le 0, u(x) \le f(x) \le v(x) \end{array}$$

$$\color{rgb(0,0,0)}x$$

$$\color{rgb(0,0,255)}f(x)$$

$$\color{rgb(127,0,0)}u(x)$$

$$\color{rgb(0,127,0)}v(x)$$