Tính bằng cách thuận tiện: 1/7 + 2/7 + 3/7 + 4/7 + 5/7 + 6/7

\[\frac{1}{7} + \frac{2}{7} + \frac{3}{7} + \frac{4}{7} + \frac{5}{7} + \frac{6}{7}\]

\[ = \left( {\frac{1}{7} + \frac{6}{7}} \right) + \left( {\frac{2}{7} + \frac{5}{7}} \right) + \left( {\frac{3}{7} + \frac{4}{7}} \right)\]

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

= 1 + 1 + 1 = 3.