Đáp án:
$\begin{array}{l}
a)\left| {1 - x} \right| - \dfrac{5}{2} = 0\\
\Leftrightarrow \left| {1 - x} \right| = \dfrac{5}{2}\\
\Leftrightarrow \left[ \begin{array}{l}
1 - x = \dfrac{5}{2} \Leftrightarrow x = 1 - \dfrac{5}{2} = - \dfrac{3}{2}\\
1 - x = - \dfrac{5}{2} \Leftrightarrow x = 1 + \dfrac{5}{2} = \dfrac{7}{2}
\end{array} \right.\\
+ Khi:x = \dfrac{{ - 3}}{2}\\
A = 3{x^2} - 5x + 1\\
= 3.{\left( {\dfrac{{ - 3}}{2}} \right)^2} - 5.\left( { - \dfrac{3}{2}} \right) + 1\\
= \dfrac{{61}}{4}\\
+ Khi:x = \dfrac{7}{2}\\
\Leftrightarrow A = 3.{\left( {\dfrac{7}{2}} \right)^2} - 5.\dfrac{7}{2} + 1 = \dfrac{{81}}{4}\\
b)B = 3.\left( {2x - 1} \right) - \left| {x - 5} \right|\\
+ Khi:x \ge 5 \Leftrightarrow \left| {x - 5} \right| = x - 5\\
\Leftrightarrow B = 6x - 3 - \left( {x - 5} \right) = 5x + 2\\
+ Khi:x < 5 \Leftrightarrow \left| {x - 5} \right| = 5 - x\\
\Leftrightarrow B = 6x - 3 - \left( {5 - x} \right) = 7x - 8
\end{array}$
