Tìm x: a) 3/7 + x = 4/5;

a) \[\frac{3}{7} + x = \frac{4}{5}\]

\[x = \frac{4}{5} - \frac{3}{7}\]

\[x = \frac{{28}}{{35}} - \frac{{15}}{{35}}\]

\[x = \frac{{13}}{{35}}\]

Vậy \[x = \frac{{13}}{{35}}\].

b) \[\left( {3x - 1} \right)\left( {\frac{{ - 1}}{2}x + 5} \right) = 0\]

• TH1: \[3x--1 = 0\]

\[3x = 1\]

\[x = \frac{1}{3}\].

• TH2: \[\frac{{ - 1}}{2}x + 5 = 0\]

\[\frac{{ - 1}}{2}x =  - 5\]

\[x =  - 5:\left( {\frac{{ - 1}}{2}} \right)\]

\[x = 10\].

Vậy \[x \in \left\{ {\frac{1}{3};\,\,10} \right\}\].