Theo Viét: $\begin{cases} x_1+x_2=-3\\ x_1.x_2=1\\\end{cases}$
`A=(\sqrt{x_1^{2}+1}-1)(\sqrt{x_{2}^2+1}-1)(\sqrt{x_1^{2}+1}+1)(\sqrt{x_2^{2}+1}+1)-2x_1^{2}-2x_2^{2}`
`A=[(\sqrt{x_1^{2}+1}-1)(\sqrt{x_1^{2}+1}+1)][(\sqrt{x_{2}^2+1}-1)(\sqrt{x_2^{2}+1}+1)]-2x_1^2-2x_2^{2}-4x_1x_2+4x_1x_2`
`A=(x_1^{2}+1-1)(x_2^{2}+1-1)-2(x_1^2+x_2^{2}+2x_1x_2)+4`
`A=x_1^{2}x_2^{2}-2(x_1+x_2)^2+4`
`A=5-2.(-3)^2`
`A=-13`
