Đáp án:
$\begin{array}{l}
B2)\\
a)\\
+ Khi:x \ge 2,2\\
\Leftrightarrow A = x - 2,2 + x - 1,8 = 2x - 4\\
+ Khi:1,8 \le x < 2,2\\
\Leftrightarrow A = 2,2 - x + x - 1,8 = 0,4\\
+ Khi:x < 1,8\\
\Leftrightarrow A = 2,2 - x + 1,8 - x = 4 - 2x\\
b)\\
B = \left| { - x - 1,4} \right| + \left| {x - 2,6} \right|\\
= \left| {x + 1,4} \right| + \left| {x - 2,6} \right|\\
+ Khi:x \ge 2,6\\
\Leftrightarrow B = x + 1,4 + x - 2,6 = 2x - 0,8\\
+ Khi: - 1,4 \le x < 2,6\\
\Leftrightarrow B = x + 1,4 + 2,6 - x = 4\\
+ Khi:x < - 1,4\\
\Leftrightarrow B = - x - 1,4 + 2,6 - x = 0,8 - 2x\\
B3)\\
a)\left| {x + \dfrac{1}{4}} \right| - \dfrac{3}{4} = 5\\
\Leftrightarrow \left| {x + \dfrac{1}{4}} \right| = 5 + \dfrac{3}{4} = \dfrac{{23}}{4}\\
\Leftrightarrow \left[ \begin{array}{l}
x + \dfrac{1}{4} = \dfrac{{23}}{4} \Leftrightarrow x = \dfrac{{11}}{2}\\
x + \dfrac{1}{4} = - \dfrac{{23}}{4} \Leftrightarrow x = - 6
\end{array} \right.\\
Vậy\,x = \dfrac{{11}}{2};x = - 6\\
b)2 - \left| {\dfrac{3}{2}x - \dfrac{1}{4}} \right| = \dfrac{5}{4}\\
\Leftrightarrow \left| {\dfrac{3}{2}x - \dfrac{1}{4}} \right| = 2 - \dfrac{5}{4} = \dfrac{3}{4}\\
\Leftrightarrow \left[ \begin{array}{l}
\dfrac{3}{2}x - \dfrac{1}{4} = \dfrac{3}{4}\\
\dfrac{3}{2}x - \dfrac{1}{4} = - \dfrac{3}{4}
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
\dfrac{3}{2}x = 1 \Leftrightarrow x = \dfrac{2}{3}\\
\dfrac{3}{2}x = - \dfrac{1}{2} \Leftrightarrow x = - 3
\end{array} \right.\\
Vậy\,x = \dfrac{2}{3};x = - 3
\end{array}$
