Tìm x , biết: (a) 2 x − 3/4 = x + 1/7 ; (b) ( 1 − 2 x ) ( 3 x + 1 ) + 3 x ( 2 x − 1 ) = 9 .

a) \(\frac{{2x - 3}}{4} = \frac{{x + 1}}{7}\)

\(7.\left( {2x - 3} \right) = 4.\left( {x + 1} \right)\)

\(14x - 21 = 4x + 4\)

\(10x = 25\)

\(x = \frac{5}{2}\)

Vậy \(x = \frac{5}{2}\).

b) \((1 - 2x)(3x + 1) + 3x(2x - 1) = 9\)

\(3x + 1 - 6{x^2} - 2x + 6{x^2} - 3x = 9\)

\[\left( { - 6{x^2} + 6{x^2}} \right) + \left( {3x - 2x - 3x} \right) = 9 - 1\]

\( - 2x = 8\)

\(x = - 4\)

Vậy \(x = - 4\).