1. Thực hiện phép tính (tính nhanh nếu có thể): a) 3/7 +- 1/5 + - 3/7; 2. Tìm \[x\]: a) 1/2 - x = - 1/6;

1.

a) \[\frac{3}{7} + \frac{{ - 1}}{5} + \frac{{ - 3}}{7}\]\[ = \left( {\frac{3}{7} + \frac{{ - 3}}{7}} \right) + \frac{{ - 1}}{5}\]\[ = 0 + \frac{{ - 1}}{5}\]\[ = \frac{{ - 1}}{5}\];                                                     

b) \[\frac{7}{{19}}.\frac{8}{{11}} + \frac{7}{{19}}.\frac{3}{{11}} + \frac{{12}}{{19}}\]\[ = \frac{7}{{19}}.\left( {\frac{8}{{11}} + \frac{3}{{11}}} \right) + \frac{{12}}{{19}}\]

\[ = \frac{7}{{19}}.\frac{{11}}{{11}} + \frac{{12}}{{19}}\]\[ = \frac{7}{{19}} + \frac{{12}}{{19}}\]\[ = \frac{{19}}{{19}} = 1\].

2.

a) \[\frac{1}{2} - x = \frac{{ - 1}}{6}\]

\[x = \frac{1}{2} - \frac{{ - 1}}{6}\]

\[x = \frac{1}{2} + \frac{1}{6}\]

\[x = \frac{2}{3}\].

Vậy \[x = \frac{2}{3}\].

b) \[50\% \] của \[x\] bằng \[\frac{1}{5}\] của 15 nên ta có:

\[50\% {\rm{ }}.{\rm{ }}x = \frac{1}{5}\,\,.\,\,15\]

\[\frac{1}{2}x = 3\]

\[x = 3:\frac{1}{2}\]

\[x = 6\].

Vậy \[x = 6\].