Thực hiện phép tính: a) 2/( - 9) - 5/(- 36) - 1/4

a) \(\frac{2}{{ - 9}} - \frac{5}{{ - 36}} - \frac{1}{4}\)

\( = \frac{{2.4}}{{\left( { - 9} \right).4}} - \frac{5}{{ - 36}} - \frac{{1.\left( { - 9} \right)}}{{4.\left( { - 9} \right)}}\)

\( = \frac{8}{{ - 36}} - \frac{5}{{ - 36}} - \frac{{ - 9}}{{ - 36}}\)

\( = \frac{{8 - 5 - \left( { - 9} \right)}}{{ - 36}}\)

\( = \frac{{12}}{{ - 36}} = \frac{{12:\left( { - 12} \right)}}{{\left( { - 36} \right):\left( { - 12} \right)}}\)

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

b) \(\left( {\frac{{ - 4}}{{ - 14}} - \frac{3}{{ - 15}}} \right) - \left( {\frac{1}{5} - \frac{{20}}{{35}} - \frac{{ - 1}}{7}} \right)\)

\( = \frac{{ - 4}}{{ - 14}} - \frac{3}{{ - 15}} - \frac{1}{5} + \frac{{20}}{{35}} + \frac{{ - 1}}{7}\)

\( = \left( {\frac{{ - 4}}{{ - 14}} + \frac{{ - 1}}{7}} \right) + \left( { - \frac{3}{{ - 15}} - \frac{1}{5} + \frac{{20}}{{35}}} \right)\)

\( = \left( {\frac{{\left( { - 4} \right):\left( { - 2} \right)}}{{\left( { - 14} \right):\left( { - 2} \right)}} + \frac{{ - 1}}{7}} \right) + \left( {\frac{{3:3}}{{15:3}} - \frac{1}{5} + \frac{{20:5}}{{35:5}}} \right)\)

\( = \left( {\frac{2}{7} + \frac{{ - 1}}{7}} \right) + \left( {\frac{1}{5} - \frac{1}{5} + \frac{4}{7}} \right)\)

\( = \frac{1}{7} + \frac{4}{7} = \frac{5}{7}\)

c) \(\frac{3}{7}.\frac{9}{{11}} + \frac{3}{7}.\frac{5}{{11}} - \frac{3}{7}.\frac{{25}}{{11}}\)

\( = \frac{3}{7}.\left( {\frac{9}{{11}} + \frac{5}{{11}} - \frac{{25}}{{11}}} \right)\)

\( = \frac{3}{7}.\left( {\frac{{9 + 5 - 25}}{{11}}} \right)\)

\( = \frac{3}{7}.\frac{{ - 11}}{{11}} = \frac{3}{7}.( - 1) = \frac{{ - 3}}{7}\)