Tìm x, biết a) \(\left( {3x + 1} \right):\left( { - 4} \right) =  - 28\)         b) \(x - \frac{4}{9} = \frac{5}{{18}}\) c) \(\frac{2}{3}x - \frac{3}{4}x = - 1\frac{1}{6...

* Hướng dẫn giải

a) \(\left( {3x + 1} \right):\left( { - 4} \right) =  - 28\)

\(\begin{array}{l}
3x + 1 =  - 28:( - 4)\\
3x + 1 = 7\\
3x = 7 - 1\\
3x = 6\\
x = 6:3 = 2
\end{array}\)

b) \(x - \frac{4}{9} = \frac{5}{{18}}\)

\(\begin{array}{l}
x = \frac{4}{9} + \frac{5}{{18}}\\
x = \frac{8}{{18}} + \frac{5}{{18}}\\
x = \frac{{13}}{{18}}
\end{array}\)

c) \(\frac{2}{3}x - \frac{3}{4}x =  - 1\frac{1}{6}\)

\(\begin{array}{l}
x\left( {\frac{2}{3} - \frac{3}{4}} \right) =  - \frac{7}{6}\\
x\left( {\frac{8}{{12}} - \frac{9}{{12}}} \right) =  - \frac{7}{6}\\
x\left( {\frac{{ - 1}}{{12}}} \right) =  - \frac{7}{6}\\
x =  - \frac{7}{6}:\left( {\frac{{ - 1}}{{12}}} \right) = \\
x =  - \frac{7}{6}.\left( { - 12} \right) = 14
\end{array}\)