Đáp án: $A = \frac{8}{{33}}$
Giải thích các bước giải:
$\begin{array}{l}
A = \frac{2}{{3.5}} + \frac{2}{{5.7}} + \frac{2}{{7.9}} + \frac{2}{{9.11}}\\
= \frac{{5 - 3}}{{3.5}} + \frac{{7 - 5}}{{5.7}} + \frac{{9 - 7}}{{7.9}} + \frac{{11 - 9}}{{9.11}}\\
= \frac{1}{3} - \frac{1}{5} + \frac{1}{5} - \frac{1}{7} + \frac{1}{7} - \frac{1}{9} + \frac{1}{9} - \frac{1}{{11}}\\
= \frac{1}{3} - \frac{1}{{11}}\\
= \frac{{11 - 3}}{{33}} = \frac{8}{{33}}
\end{array}$
