\[\begin{array}{l}
a)\,\,T = {x_1} + 2{x_2} = 7\\
Ap\,\,dung\,\,voi\,\,\,{x_1} + {x_2} = - \frac{b}{a}\,\,\,de\,\,tim\,\,{x_1},\,\,{x_2}.\\
b)\,\,\,T = x_a^2 + x_b^2 = {\left( {{x_a} + {x_b}} \right)^2} - 2{x_a}{x_b}.\\
c)\,\,\,T = \left| {{x_1} - {x_2}} \right|\\
\Rightarrow {T^2} = {\left( {{x_1} - {x_2}} \right)^2} = {\left( {{x_1} + {x_2}} \right)^2} - 4{x_1}{x_2}.\\
d)\,\,T = x_1^2 + x_2^2 - 10\left( {{x_1} + {x_2}} \right)\\
= {\left( {{x_1} + {x_2}} \right)^2} - 2{x_1}{x_2} - 10\left( {{x_1} + {x_2}} \right).
\end{array}\]