Tìm x biết:a) (left( {3x + 1} ight):left( { - 4} ight) =  - 28)b) (x - frac{4}{9} = frac{5}{{18}})c) (frac{

* Hướng dẫn giải

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

Vậy x = 2

b) Ta có

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

Vậy \9x = \frac{{13}}{{18}}\)

c) 

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

Vậy x = 14