Tìm giá trị của x , biết: (a) x − 3/7 = 3/10 ; (b) ( 42 x^3 − 12 x ) : ( − 6 x ) + 7 x ( x + 2 ) = 8 .

a) \(\frac{{x - 3}}{7} = \frac{3}{{10}}\)

\(10.\left( {x - 3} \right) = 7.3\)

\(10x - 30 = 21\)

\(10x = 51\)

\(x = \frac{{51}}{{10}}\)

Vậy \(x = \frac{{51}}{{10}}\).

b) \(\left( {42{x^3} - 12x} \right):\left( { - 6x} \right) + 7x\left( {x + 2} \right) = 8\)

\(\left( {42{x^3}} \right):\left( { - 6x} \right) - \left( {12x} \right):\left( { - 6x} \right) + 7{x^2} + 14x = 8\)

\( - 7{x^2} + 2 + 7{x^2} + 14x = 8\)

\(14x = 6\)

\(x = \frac{3}{7}\)

Vậy \(x = \frac{3}{7}\).