Tính tổng: 2/(3x5) 2/(5x7) … 2/(79x81)

Lời giải:

\[A = \frac{2}{{3 \times 5}} + \frac{2}{{5 \times 7}} + \frac{2}{{7 \times 9}} + ... + \frac{2}{{97 \times 99}}\]

\[ = \frac{1}{3} - \frac{1}{5} + \frac{1}{5} - \frac{1}{7} + \frac{1}{7} - \frac{1}{9} + ... + \frac{1}{{97}} - \frac{1}{{99}}\]

\[ = \frac{1}{3} - \frac{1}{{99}}\]

\[ = \frac{{32}}{{99}}.\]