Tính tổng:\[\frac{6}{8} + \frac{6}{{56}} + \frac{6}{{140}} + ... + \frac{6}{{1100}} + \frac{6}{{1400}}\].

\[\begin{array}{l}\frac{6}{8} + \frac{6}{{56}} + \frac{6}{{140}} + ... + \frac{6}{{1100}} + \frac{6}{{1400}}\\ = \frac{3}{4} + \frac{3}{{28}} + \frac{3}{{140}} + ... + \frac{3}{{550}} + \frac{3}{{700}}\\ = \frac{3}{{1.4}} + \frac{3}{{4.7}} + \frac{3}{{7.10}} + ... + \frac{3}{{22.25}} + \frac{3}{{25.28}}\\ = 1 - \frac{1}{4} + \frac{1}{4} - \frac{1}{7} + \frac{1}{7} - \frac{1}{{10}} + ... + \frac{1}{{22}} - \frac{1}{{25}} + \frac{1}{{25}} - \frac{1}{{28}}\\ = 1 - \frac{1}{{28}}\\ = \frac{{28}}{{28}} - \frac{1}{{28}}\\ = \frac{{27}}{{28}}\end{array}\]