Tìm giá trị x , biết: (a) x − 1/6 = x − 5/7 ; (b) ( 1 − 2 x ) ( 3 x + 1 ) + 3 x ( 2 x − 1 ) = 9 .

a) \[\frac{{x - 1}}{6} = \frac{{x - 5}}{7}\]

\(7.\left( {x - 1} \right) = 6.\left( {x - 5} \right)\)

\(7x - 7 = 6x - 30\)

\(x = 37\)

Vậy \(x = 37\).

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 \(x = - 4\).