Ta có: [tex]\left\{\begin{matrix} a+\sqrt{1+a^2}=\frac{4}{\sqrt{1+b^2}+b}=4(\sqrt{1+b^2}-b)\\ b+\sqrt{1+b^2}=\frac{4}{\sqrt{1+a^2}+a}=4(\sqrt{1+a^2}-a) \end{matrix}\right.\Rightarrow a+b+(\sqrt{1+a^2}+\sqrt{1+b^2})=4(\sqrt{1+a^2}+\sqrt{1+b^2})-4(a+b)\Rightarrow 5(a+b)=3(\sqrt{1+a^2}+\sqrt{1+b^2}) \geq 3\sqrt{(1+1)^2+(a+b)^2}=3\sqrt{(a+b)^2+4} > 0 \Rightarrow 25(a+b)^2 \geq 9[(a+b)^2+4] \Rightarrow 16(a+b)^2 \geq 36 \Rightarrow a+b \geq \frac{3}{2}[/tex]