Tính bằng cách thuận tiện.

\(\frac{4}{5}\,\, \times \,\,\frac{6}{7}\,\, + \,\,\frac{4}{5}\,\, \times \,\,\frac{3}{7}\)

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

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

\( = \frac{4}{5}\, \times \,\frac{9}{7}\)

\( = \frac{{36}}{{35}}\,\)

\(\frac{5}{9}\,\, + \,\,\frac{5}{7}\,\, + \,\,\frac{9}{7}\,\, + \,\,\frac{4}{9}\)

\( = \left( {\frac{5}{9}\,\, + \frac{4}{9}} \right)\, + \,\left( {\frac{5}{7}\,\, + \,\,\frac{9}{7}} \right)\,\,\,\,\)

\( = \frac{9}{9}\, + \,\frac{{14}}{7}\,\,\,\)

= 1 + 2

= 3