Tính

a) \[\frac{2}{7}\,\, + \,\,\frac{7}{6}\,\, = \,\,\frac{{12}}{{42}}\,\, + \,\,\frac{{49}}{{42}}\,\, = \,\,\frac{{61}}{{42}}\]

b) \[\frac{7}{8}\,\, - \,\,\frac{4}{7}\,\, = \,\,\frac{{49}}{{56}}\,\, - \,\,\frac{{32}}{{56}}\,\, = \,\,\frac{{17}}{{56}}\]

c) \[\frac{7}{9}\,\, \times \,\,\frac{{81}}{{49}}\,\, = \,\,\frac{{\cancel{7}\,\, \times \,\,\cancel{9}\,\, \times \,\,9}}{{\cancel{9}\,\, \times \,\,\cancel{7}\,\, \times \,\,7}}\,\, = \,\,\frac{9}{7}\]

d) \[\frac{{35}}{{49}}\,\,:\,\,\frac{5}{7}\,\, = \,\,\frac{{35}}{{49}}\,\, \times \,\,\frac{7}{5}\,\, = \,\,\frac{5}{7}\,\, \times \,\,\frac{7}{5}\,\, = \,\,1\]