Đáp án:
$\begin{array}{l}
\dfrac{{{x_1} - 1}}{{{x_2} + 1}} + \dfrac{{{x_2} - 1}}{{{x_1} + 1}}\\
= \dfrac{{\left( {{x_1} - 1} \right)\left( {{x_1} + 1} \right) + \left( {{x_2} - 1} \right)\left( {{x_2} + 1} \right)}}{{\left( {{x_2} + 1} \right)\left( {{x_1} + 1} \right)}}\\
= \dfrac{{x_1^2 - 1 + x_2^2 - 1}}{{{x_1}{x_2} + {x_1} + {x_2} + 1}}\\
= \dfrac{{{{\left( {{x_1} + {x_2}} \right)}^2} - 2{x_1}{x_2} - 2}}{{12 + 7 + 1}}\\
= \dfrac{{{7^2} - 2.12 - 2}}{{20}}\\
= \dfrac{{49 - 26}}{{20}}\\
= \dfrac{{23}}{{20}}
\end{array}$
