Đáp án:
$\begin{array}{l}
a)\dfrac{{ - 3}}{4} + \dfrac{3}{7} + \dfrac{{ - 1}}{4} + \dfrac{4}{9} + \dfrac{4}{7}\\
= \dfrac{{ - 3}}{4} + \dfrac{{ - 1}}{4} + \dfrac{3}{7} + \dfrac{4}{7} + \dfrac{4}{9}\\
= \dfrac{{ - 4}}{4} + \dfrac{7}{7} + \dfrac{4}{9}\\
= - 1 + 1 + \dfrac{4}{9}\\
= \dfrac{4}{9}\\
b)\dfrac{{ - 7}}{9}.\dfrac{4}{{11}} + \dfrac{{ - 7}}{9}.\dfrac{7}{{11}} + \dfrac{{52}}{9}\\
= - \dfrac{7}{9}.\left( {\dfrac{4}{{11}} + \dfrac{7}{{11}}} \right) + \dfrac{{52}}{9}\\
= - \dfrac{7}{9}.\dfrac{{11}}{{11}} + \dfrac{{52}}{9}\\
= - \dfrac{7}{9} + \dfrac{{52}}{9}\\
= \dfrac{{45}}{9}\\
= 5\\
c)\left( {\dfrac{{21}}{{31}} + \dfrac{{2013}}{{6039}}} \right) - \left( {\dfrac{{44}}{{53}} - \dfrac{{10}}{{31}}} \right) - \dfrac{9}{{53}}\\
= \dfrac{{21}}{{31}} + \dfrac{1}{3} - \dfrac{{44}}{{53}} + \dfrac{{10}}{{31}} - \dfrac{9}{{53}}\\
= \left( {\dfrac{{21}}{{31}} + \dfrac{{10}}{{31}}} \right) + \dfrac{1}{3} + \left( { - \dfrac{{44}}{{53}} - \dfrac{9}{{53}}} \right)\\
= \dfrac{{31}}{{31}} + \dfrac{1}{3} - \dfrac{{53}}{{53}}\\
= 1 + \dfrac{1}{3} - 1\\
= \dfrac{1}{3}
\end{array}$
