Đáp án:
$\begin{array}{l}
2)a)\dfrac{2}{9} - \left( {\dfrac{1}{3} - x} \right) = \dfrac{5}{6}\\
\Leftrightarrow \dfrac{2}{9} - \dfrac{1}{3} + x = \dfrac{5}{6}\\
\Leftrightarrow x = \dfrac{5}{6} - \dfrac{2}{9} + \dfrac{1}{3}\\
\Leftrightarrow x = \dfrac{{5.3 - 2.2 + 6}}{{18}}\\
\Leftrightarrow x = \dfrac{{17}}{{18}}\\
Vậy\,x = \dfrac{{17}}{{18}}\\
b)\left| {x - \dfrac{1}{4}} \right| + 0,75 = 1,75\\
\Leftrightarrow \left| {x - \dfrac{1}{4}} \right| = 1\\
\Leftrightarrow \left[ \begin{array}{l}
x - \dfrac{1}{4} = 1 \Leftrightarrow x = 1 + \dfrac{1}{4} = \dfrac{5}{4}\\
x - \dfrac{1}{4} = - 1 \Leftrightarrow x = - 1 + \dfrac{1}{4} = \dfrac{{ - 3}}{4}
\end{array} \right.\\
Vậy\,x = \dfrac{5}{4};x = - \dfrac{3}{4}\\
c)\left( {x - 5} \right)\left( {{x^2} + 3} \right).\left| {x + 1} \right| < 0\\
\Leftrightarrow \left\{ \begin{array}{l}
x - 5 < 0\\
x + 1 \ne 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
x < 5\\
x \ne - 1
\end{array} \right.\\
Vậy\,x < 5;x \ne - 1\\
3)2)\dfrac{{ - 24}}{a}\\
\dfrac{{ - 6}}{7} < \dfrac{{ - 24}}{a} < \dfrac{{ - 4}}{5}\\
\Leftrightarrow \dfrac{{ - 24}}{{28}} < \dfrac{{ - 24}}{a} < \dfrac{{ - 24}}{{30}}\\
\Leftrightarrow 28 < a < 30\\
\Leftrightarrow a = 29\\
\Leftrightarrow \dfrac{{ - 24}}{{29}}
\end{array}$
