Thực hiện các phép tính (Tính hợp lí nếu có thể): a) \(\frac{{ - 17}}{{15}} + \frac{{11}}{9} - \frac{{24}}{{15}} - \frac{2}{9} + \frac{{11}}{{15}} - \frac{1}{3}:\frac{2}{{27}}.\)     b) \(\

a) \(\frac{{ - 17}}{{15}} + \frac{{11}}{9} - \frac{{24}}{{15}} - \frac{2}{9} + \frac{{11}}{{15}} - \frac{1}{3}:\frac{2}{{27}}\)

\( = \left( {\frac{{ - 17}}{{15}} - \frac{{24}}{{15}} + \frac{{11}}{{15}}} \right) + \left( {\frac{{11}}{9} - \frac{2}{9}} \right) - \frac{1}{3}.\frac{{27}}{2}\)

\( = - 2 + 1 - \frac{9}{2}\)

\( = - 1 - \frac{9}{2} = \frac{{ - 11}}{2}.\)

b) \(\frac{4}{{3.7}} + \frac{4}{{7.11}} + \frac{4}{{11.15}} + \frac{4}{{15.19}}\)

\( = \frac{1}{3} - \frac{1}{7} + \frac{1}{7} - \frac{1}{{11}} + \frac{1}{{11}} - \frac{1}{{15}} + \frac{1}{{15}} - \frac{1}{{19}}\)

\( = \frac{1}{3} - \frac{1}{{19}}\)

\( = \frac{{16}}{{57}}\).