Đáp án:
15) \(\left[ \begin{array}{l}
x = \dfrac{5}{4}\\
x = - 2
\end{array} \right.\)
Giải thích các bước giải:
\(\begin{array}{l}
11)18{x^2} + 24x - 21x - 28 = - 6{x^2} + 6x + 7x - 7\\
\to 24{x^2} - 10x - 21 = 0\\
\to 24{x^2} + 18x - 28x - 21 = 0\\
\to 6x\left( {4x + 3} \right) - 7\left( {4x + 3} \right) = 0\\
\to \left( {4x + 3} \right)\left( {6x - 7} \right) = 0\\
\to \left[ \begin{array}{l}
x = - \dfrac{3}{4}\\
x = \dfrac{7}{6}
\end{array} \right.\\
12)\dfrac{4}{5}x - 1 = \dfrac{1}{5}x.4x - \dfrac{1}{5}x.5\\
\to \dfrac{4}{5}{x^2} - \dfrac{9}{5}x + 1 = 0\\
\to 4{x^2} - 9x + 5 = 0\\
\to \left( {4x - 5} \right)\left( {x - 1} \right) = 0\\
\to \left[ \begin{array}{l}
x = \dfrac{5}{4}\\
x = 1
\end{array} \right.\\
15)\left( {4x - 5} \right)\left( {4x + 5} \right) - \left( {4x - 5} \right)\left( {2x + 1} \right) = 0\\
\to \left( {4x - 5} \right)\left( {4x + 5 - 2x - 1} \right) = 0\\
\to \left[ \begin{array}{l}
x = \dfrac{5}{4}\\
2x + 4 = 0
\end{array} \right.\\
\to \left[ \begin{array}{l}
x = \dfrac{5}{4}\\
x = - 2
\end{array} \right.
\end{array}\)
