Tính bằng cách thuận tiện.

\[\frac{4}{9}\,\, + \,\frac{5}{7}\,\, + \,\frac{5}{9}\,\, + \,\,\frac{2}{7}\] ………………………………………. ………………………………………. ………………………………………. ………………………………………. ………………………………………. ……………………………………….

\[\frac{4}{7}\,\, \times \,\,\frac{4}{9}\, + \,\frac{4}{7}\,\, \times \,\,\frac{1}{3}\,\, + \,\,\frac{4}{7}\,\, \times \,\,\frac{2}{9}\] ………………………………………. ………………………………………. ………………………………………. ………………………………………. ………………………………………. ……………………………………….

\[\frac{4}{9}\,\, + \,\frac{5}{7}\,\, + \,\frac{5}{9}\,\, + \,\,\frac{2}{7}\]

\[ = \,\,\left( {\frac{4}{9}\,\, + \,\,\frac{5}{9}} \right)\,\, + \,\,\left( {\frac{5}{7}\,\, + \,\,\frac{2}{7}} \right)\]

\[ = \,\,\frac{9}{9}\,\, + \,\,\frac{7}{7}\]

= 1 + 1 = 2

\[\frac{4}{7}\,\, \times \,\,\frac{4}{9}\, + \,\frac{4}{7}\,\, \times \,\,\frac{1}{3}\,\, + \,\,\frac{4}{7}\,\, \times \,\,\frac{2}{9}\]

\[ = \,\,\frac{4}{7}\,\, \times \,\,\left( {\frac{4}{9}\,\, + \,\,\frac{1}{3}\,\, + \,\,\frac{2}{9}} \right)\]

\[ = \,\,\frac{4}{7}\,\, \times \,\,\left( {\frac{4}{9}\,\, + \,\,\frac{3}{9}\,\, + \,\,\frac{2}{9}} \right)\]

\[ = \,\,\frac{4}{7}\,\, \times \,\,\frac{9}{9} = \,\,\frac{4}{7}\,\, \times \,1 = \,\,\frac{4}{7}\]