f:(1,3)→R,f(x)=1+x2x[x]
f(x)={\begin{matrix}\frac{x}{1+{x}^{2}}, & x\in (1,2) \\ \frac{2x}{1+{x}^{2}}, & x\in [2,3)\end{matrix}
{f}^{'}(x)={\begin{matrix}\frac{(1+{x}^{2})(1)-x(2x)}{{(1+{x}^{2})}^{2}}, & x\in (1,2) \\ \frac{(1+{x}^{2})(2)-2x(2x)}{{(1+{x}^{2})}^{2}}, & x\in [2,3)\end{matrix}
{f}^{'}(x)={\begin{matrix}\frac{1-{x}^{2}}{1+{x}^{2}}, & x\in (1,2) \\ \frac{1-2{x}^{2}}{1+{x}^{2}}, & x\in [2,3)\end{matrix}
∴f(x) is decreasing function.
∴Rf∈(52,21)∪(106,54]