Thực hiện phép tính (tính hợp lí nếu có thể): a) - 3,1 - 99 + 2,1; b) 1/8- 9/8.4/3

a) \( - 3,1 - 99 + 2,1\)

\( = \left( { - 3,1 + 2,1} \right) - 99\)

\( =  - 1 - 99\)

\( =  - 100\).

c) \(1\frac{{13}}{{15}}.0,75 - \left( {\frac{{11}}{{20}} + 25\% } \right):\frac{5}{7}\)

\( = \frac{{28}}{{15}}.\frac{3}{4} - \left( {\frac{{11}}{{20}} + \frac{1}{4}} \right).\frac{7}{5}\)

\( = \frac{7}{5} - \left( {\frac{{11}}{{20}} + \frac{5}{{20}}} \right).\frac{7}{5}\)

\( = \frac{7}{5} - \frac{{16}}{{20}}.\frac{7}{5}\)

\( = \frac{7}{5}.\left( {1 - \frac{{16}}{{20}}} \right)\)

\( = \frac{7}{5}.\frac{4}{{20}} = \frac{7}{{25}}\).

b) \(\frac{1}{8} - \frac{9}{8}.\frac{4}{3}\)

\( = \frac{1}{8} - \frac{3}{2}\)

\( = \frac{1}{8} - \frac{{12}}{8}\)

\( = \frac{{ - 11}}{8}\).

d) \[\frac{{ - 3}}{4}.\frac{{2022}}{{2023}} - \frac{{2022}}{{2023}}.\frac{1}{4}\]

\[ = \frac{{2022}}{{2023}}.\left( {\frac{{ - 3}}{4} - \frac{1}{4}} \right)\]

\[ = \frac{{2022}}{{2023}}.\left( { - 1} \right)\]

\[ =  - \frac{{2022}}{{2023}}\].