Thực hiện phép tính (tính hợp lí nếu có thể): a)4/5+3/5.(- 2/15); b)- 5/17.31/33 -5/17.2/33 + 1 5/17

a) \(\frac{4}{5} + \frac{3}{5}.\left( { - \frac{2}{{15}}} \right)\)

\( = \frac{4}{5} + \frac{{ - 2}}{{25}}\)

\( = \frac{{20}}{{25}} + \frac{{ - 2}}{{25}}\)

\( = \frac{{18}}{{25}}\);

b) \(\frac{{ - 5}}{{17}}.\frac{{31}}{{33}} - \frac{5}{{17}}.\frac{2}{{33}} + 1\frac{5}{{17}}\)

\( = \frac{{ - 5}}{{17}}.\left( {\frac{{31}}{{33}} + \frac{2}{{33}}} \right) + \frac{{22}}{{17}}\)

\( = \frac{{ - 5}}{{17}}.1 + \frac{{22}}{{17}}\)

\( = \frac{{17}}{{17}} = 1\).

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

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

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

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

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

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