Đáp án:
Giải thích các bước giải:
\[\begin{array}{l}
2.a)\,\frac{5}{6} = \frac{2}{6} + \frac{3}{6} = \frac{1}{3} + \frac{1}{2}\\
\,\,\,\,b)\,\frac{7}{8} = \frac{1}{8} + \frac{2}{8} + \frac{4}{8} = \frac{1}{8} + \frac{1}{4} + \frac{1}{2}\\
3.\\
\,A = \frac{1}{{2 \times 3}} + \frac{1}{{3 \times 4}} + \frac{1}{{4 \times 5}} + \frac{1}{{5 \times 6}} + \frac{1}{{6 \times 7}} + \frac{1}{{7 \times 8}}\\
\,\,\,\,\,\, = \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + \frac{1}{4} - \frac{1}{5} + \frac{1}{5} - \frac{1}{6} + \frac{1}{6} - \frac{1}{7} + \frac{1}{7} - \frac{1}{8}\\
\,\,\,\,\,\, = \frac{1}{2} - \frac{1}{8}\\
\,\,\,\,\,\, = \,\frac{{4 - 1}}{8} = \frac{3}{8}
\end{array}\]
