Thực hiện phép tính (tính hợp lí nếu có thể):a) (- 0,4).(- 2,5).(- 0,8); b)5/7 +4/- 14

a) \(\left( { - 0,4} \right).\left( { - 2,5} \right).\left( { - 0,8} \right)\)

\( = 1.\left( { - 0,8} \right)\)\( =  - 0,8\).

c) \(\left( { - 2,4 + \frac{1}{3}} \right):3,1 + 75\% :1\frac{1}{2}\)

\[ = \left( { - \frac{{24}}{{10}} + \frac{1}{3}} \right):\frac{{31}}{{10}} + \frac{{75}}{{100}}:\frac{3}{2}\]

\[ = \left( { - \frac{{12}}{5} + \frac{1}{3}} \right).\frac{{10}}{{31}} + \frac{3}{4}.\frac{2}{3}\]

\[ = \frac{{ - 31}}{{15}}.\frac{{10}}{{31}} + \frac{1}{2}\]

\[ = \frac{{ - 2}}{3} + \frac{1}{2}\]

\[ = \frac{{ - 4}}{6} + \frac{3}{6} =  - \frac{1}{6}\].

b) \[\frac{5}{7} + \frac{4}{{ - 14}}\]

\[ = \frac{5}{7} + \frac{{ - 2}}{7}\]\[ = \frac{3}{7}\].

d) \(\frac{2}{{11}}.\frac{{ - 5}}{4} + \frac{{ - 9}}{{11}}.\frac{5}{4} + 1\frac{3}{4}\)

\( = \frac{2}{{11}}.\frac{{ - 5}}{4} + \frac{9}{{11}}.\frac{{ - 5}}{4} + \frac{7}{4}\)

\( = \frac{{ - 5}}{4}.\left( {\frac{2}{{11}} + \frac{9}{{11}}} \right) + \frac{7}{4}\)

\( = \frac{{ - 5}}{4}.1 + \frac{7}{4}\)

\[ = \frac{5}{4} + \frac{7}{4}\]

\[ = \frac{{12}}{4} = 3\].