Tính bằng cách thuận tiện.

\[\frac{6}{{11}}\,\, + \,\,\frac{3}{7}\,\, + \,\,\frac{5}{{11}}\,\, + \,\,\frac{4}{7}\]

\[ = \,\,\left( {\frac{6}{{11}}\,\, + \,\,\frac{5}{{11}}} \right)\,\, + \,\,\left( {\frac{3}{7}\,\, + \,\,\frac{4}{7}} \right)\]

\[ = \,\,\frac{{11}}{{11}}\,\, + \,\,\frac{7}{7}\]

= 1 + 1 = 2

\[\frac{5}{9}\,\, \times \,\,\frac{{11}}{{35}}\,\, + \,\,\frac{7}{{35}}\,\,\, \times \,\,\frac{5}{9}\]

\[ = \,\,\frac{5}{9}\,\, \times \,\,\left( {\frac{{11}}{{35}}\,\, + \,\,\frac{7}{{35}}} \right)\]

\[ = \,\,\frac{5}{9}\,\, \times \,\,\frac{{18}}{{35}}\]

\[ = \,\,\frac{2}{7}\]