1. Thực hiện phép tính (tính nhanh nếu có thể): a) (2/3 + 5/7 + - 2/3); 2 . Tìm x: a) 2/3:x = 2,4 - 4/5; 

1.

a) \(\frac{2}{3} + \left( {\frac{5}{7} + \frac{{ - 2}}{3}} \right)\)\( = \frac{2}{3} + \frac{5}{7} + \frac{{ - 2}}{3}\)

\[ = \left( {\frac{2}{3} + \frac{{ - 2}}{3}} \right) + \frac{5}{7}\]\( = 0 + \frac{5}{7} = \frac{5}{7}\);

b) \(\frac{5}{{17}}.\frac{{ - 7}}{3} + \frac{8}{{17}}.\frac{{ - 7}}{3} + \frac{{ - 7}}{3}.\frac{4}{{17}}\)

\[ = \frac{{ - 7}}{3}.\left( {\frac{5}{{17}} + \frac{8}{{17}} + \frac{4}{{17}}} \right)\]

\[ = \frac{{ - 7}}{3}.1 =  - \frac{7}{3}\].

2.

a) \[\frac{2}{3}:x = 2,4 - \frac{4}{5}\] \[\frac{2}{3}:x = \frac{{12}}{5} - \frac{4}{5}\] \[\frac{2}{3}:x = \frac{8}{5}\] \[x = \frac{2}{3}:\frac{8}{5}\] \[x = \frac{5}{{12}}\]. Vậy \[x = \frac{5}{{12}}\].

b) \[\frac{5}{4}.\left( {x - \frac{3}{5}} \right) = \frac{{ - 1}}{8}\] \[x - \frac{3}{5} = \frac{{ - 1}}{8}:\frac{5}{4}\] \[x - \frac{3}{5} = \frac{{ - 1}}{{10}}\] \[x = \frac{{ - 1}}{{10}} + \frac{3}{5}\] \[x = \frac{1}{2}\]. Vậy \[x = \frac{1}{2}\].