Thực hiện phép tính (tính hợp lí nếu có thể): a) 2/3 + (5/7 + - 2/3); b) 4,5.[- 4/45: (20%  - 2/15) + 1 2/3)]

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

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

\( = 0 + \frac{5}{7}\)\( = \frac{5}{7}\).

c) \(8,712.5,18 + 5,18.2,788 + 11,5.4,82\)

\[ = 5,18.\left( {8,712 + 2,788} \right) + 11,5.4,82\]

\[ = 5,18.11,5 + 11,5.4,82\]

\[ = 11,5.\left( {5,18 + 4,82} \right)\]

\[ = 11,5.10\]

\( = 115\)

b) \(4,5.\left[ { - \frac{4}{{45}}:\left( {20\%  - \frac{2}{{15}}} \right) + 1\frac{2}{3}} \right]\)

\( = \frac{9}{2}.\left[ { - \frac{4}{{45}}:\left( {\frac{1}{5} - \frac{2}{{15}}} \right) + \frac{5}{3}} \right]\)

\( = \frac{9}{2}.\left[ { - \frac{4}{{45}}:\frac{1}{{15}} + \frac{5}{3}} \right]\)

\( = \frac{9}{2}.\left[ { - \frac{4}{{45}}.15 + \frac{5}{3}} \right]\)

\( = \frac{9}{2}.\left[ { - \frac{4}{3} + \frac{5}{3}} \right]\)

\( = \frac{9}{2}.\frac{1}{3}\)

\( = \frac{3}{2}\).