Đáp án:
$\begin{array}{l}
1)\dfrac{{11}}{{12}} - \left( {\dfrac{2}{5} + x} \right) = \dfrac{2}{3}\\
\Rightarrow \dfrac{{11}}{{12}} - \dfrac{2}{5} - x = \dfrac{2}{3}\\
\Rightarrow x = \dfrac{{11}}{{12}} - \dfrac{2}{5} - \dfrac{2}{3}\\
\Rightarrow x = \dfrac{{11.5 - 2.12 - 2.20}}{{60}}\\
\Rightarrow x = \dfrac{{ - 9}}{{60}}\\
\Rightarrow x = \dfrac{{ - 3}}{{20}}\\
\text{Vậy}\,x = \dfrac{{ - 3}}{{20}}\\
2)7\left( {x - 1} \right) + 2x\left( {1 - x} \right) = 0\\
\Rightarrow 7\left( {x - 1} \right) - 2x\left( {x - 1} \right) = 0\\
\Rightarrow \left( {x - 1} \right)\left( {7 - 2x} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
x - 1 = 0\\
7 - 2x = 0
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 1\\
x = \dfrac{7}{2}
\end{array} \right.\\
\text{Vậy}\,x = 1\,\text{hoặc}\,x = \dfrac{7}{2}\\
3)\dfrac{3}{4} + \dfrac{1}{4}:x = \dfrac{2}{5}\\
\Rightarrow \dfrac{1}{4}:x = \dfrac{2}{5} - \dfrac{3}{4}\\
\Rightarrow \dfrac{1}{4}:x = \dfrac{{ - 7}}{{20}}\\
\Rightarrow x = \dfrac{1}{4}:\dfrac{{ - 7}}{{20}}\\
\Rightarrow x = \dfrac{{ - 5}}{7}\\
\text{Vậy}\,x = - \dfrac{5}{7}
\end{array}$
