Đáp án:
$\begin{array}{l}
5)5x\left( {\dfrac{1}{5}x - 2} \right) + 3\left( {6 - \dfrac{1}{3}{x^2}} \right) = 12\\
\Leftrightarrow {x^2} - 10x + 18 - {x^2} - 12 = 0\\
\Leftrightarrow 10x = 6\\
\Leftrightarrow x = \dfrac{3}{5}\\
Vậy\,x = \dfrac{3}{5}\\
6)3x\left( {\dfrac{4}{3}x + 1} \right) - 4x\left( {x - 2} \right) = 10\\
\Leftrightarrow 4{x^2} + 3x - 4{x^2} + 8x = 10\\
\Leftrightarrow 11x = 10\\
\Leftrightarrow x = \dfrac{{10}}{{11}}\\
Vậy\,x = \dfrac{{10}}{{11}}\\
7)5\left( {{x^2} - 3x + 1} \right) + x\left( {1 - 5x} \right) = x - 2\\
\Leftrightarrow 5{x^2} - 15x + 5 + x - 5{x^2} - x = - 2\\
\Leftrightarrow - 15x = - 7\\
\Leftrightarrow x = \dfrac{7}{{15}}\\
Vậy\,x = \dfrac{7}{{15}}\\
8)12{x^2} - 4x\left( {3x - 5} \right) = 10x - 17\\
\Leftrightarrow 12{x^2} - 12{x^2} + 20x = 10x - 17\\
\Leftrightarrow 10x = - 17\\
\Leftrightarrow x = \dfrac{{ - 17}}{{10}}\\
Vậy\,x = \dfrac{{ - 17}}{{10}}\\
9)4{x^2} - 2x + 3 - 4x\left( {x - 5} \right) = 7x - 3\\
\Leftrightarrow 4{x^2} - 2x + 3 - 4{x^2} + 20x - 7x = - 3\\
\Leftrightarrow 11x = - 6\\
\Leftrightarrow x = \dfrac{{ - 6}}{{11}}\\
Vậy\,x = \dfrac{{ - 6}}{{11}}\\
10) - 3x\left( {x - 5} \right) + 5\left( {x - 1} \right) + 3{x^2} = 4 - x\\
\Leftrightarrow - 3{x^2} + 15x + 5x - 5 + 3{x^2} = 4 - x\\
\Leftrightarrow 20x + x = 4 + 5\\
\Leftrightarrow 21x = 9\\
\Leftrightarrow x = \dfrac{3}{7}\\
Vậy\,x = \dfrac{3}{7}\\
11)\dfrac{1}{5}x\left( {10x - 15} \right) - 2x\left( {x - 5} \right) = 12\\
\Leftrightarrow 2{x^2} - 3x - 2{x^2} + 10x = 12\\
\Leftrightarrow 7x = 12\\
\Leftrightarrow x = \dfrac{{12}}{7}\\
Vậy\,x = \dfrac{{12}}{7}\\
12)3\left( {5x - 1} \right) - x\left( {x - 2} \right) + {x^2} - 13x = 7\\
\Leftrightarrow 15x - 3 - {x^2} + 2x + {x^2} - 13x = 7\\
\Leftrightarrow 4x = 10\\
\Leftrightarrow x = \dfrac{5}{2}\\
Vậy\,x = \dfrac{5}{2}
\end{array}$
