Thực hiện phép tính: (a) 2/9 + 5/12 − − 3/4 .

a) \(\frac{2}{9} + \frac{5}{{12}} - \frac{{ - 3}}{4}\)

\( = \frac{8}{{36}} + \frac{{15}}{{36}} - \frac{{ - 27}}{{36}}\)

\( = \frac{{50}}{{36}} = \frac{{25}}{{18}}.\)

b) \(\frac{{ - 2}}{3} \cdot \frac{5}{{11}} + \frac{{ - 2}}{3} \cdot \frac{2}{{11}} + \frac{{ - 2}}{3} \cdot \frac{4}{{11}}\)

\( = \frac{{ - 2}}{3} \cdot \left( {\frac{5}{{11}} + \frac{2}{{11}} + \frac{4}{{11}}} \right)\)

\[ = \frac{{ - 2}}{3} \cdot \frac{{11}}{{11}} = \frac{{ - 2}}{3}.\]

c) \(\frac{{ - 3}}{8} + \frac{3}{5}:\frac{7}{{10}}\)

\( = \frac{{ - 3}}{8} + \frac{3}{5} \cdot \frac{{10}}{7}\)

\( = \frac{{ - 3}}{8} + \frac{6}{7}\)\( = \frac{{ - 21}}{{56}} + \frac{{48}}{{56}}\)

\( = \frac{{27}}{{56}}.\)