Thực hiện phép tính một cách hợp lí. a) \(\frac{5}{3}.\frac{{ - 8}}{{25}} + \frac{5}{3}.\frac{{ - 21}}{{25}} + \frac{5}{3}.\frac{4}{{25}} - \frac{3}{5}.\)       b) \(\frac{6}{{13}}.\frac{{

a) \(\frac{5}{3}.\frac{{ - 8}}{{25}} + \frac{5}{3}.\frac{{ - 21}}{{25}} + \frac{5}{3}.\frac{4}{{25}} - \frac{3}{5}.\)

\( = \frac{5}{3}.\left( {\frac{{ - 8}}{{25}} + \frac{{ - 21}}{{25}} + \frac{4}{{25}}} \right) - \frac{3}{5}\)

\( = \frac{5}{3}.\left( {\frac{{ - 25}}{{25}}} \right) - \frac{3}{5}\)

\( = \frac{5}{3}.\left( { - 1} \right) - \frac{3}{5}\)

\( = \frac{{ - 5}}{3} - \frac{3}{5}\)

\( = \frac{{ - 25}}{{15}} - \frac{9}{{15}}\)

\( = \frac{{ - 34}}{{15}}.\)

b) \(\frac{6}{{13}}.\frac{{ - 11}}{{21}} + \frac{6}{{13}}.\frac{{ - 15}}{{21}} + \frac{6}{{13}}.\frac{5}{{21}} - \frac{{13}}{6}\)

\( = \frac{6}{{13}}.\left( {\frac{{ - 11}}{{21}} + \frac{{ - 15}}{{21}} + \frac{5}{{21}}} \right) - \frac{{13}}{6}\)

\( = \frac{6}{{13}}.\left( {\frac{{ - 21}}{{21}}} \right) - \frac{{13}}{6}\)

\( = \frac{6}{{13}}.\left( { - 1} \right) - \frac{{13}}{6}\)

\( = \frac{{ - 6}}{{13}} - \frac{{13}}{6}\)

\( = \frac{{ - 36}}{{78}} - \frac{{169}}{{78}}\)

\( = \frac{{ - 205}}{{78}}\).