A

C

A

D

C

thế này :

= \(\frac{1}{2}\left(\frac{2}{3.5}+\frac{2}{5.7}+......+\frac{2}{11.13}\right)\)

= \(\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+....+\frac{1}{11}-\frac{1}{13}\right)\)

= \(\frac{1}{2}\left(\frac{1}{3}-\frac{1}{13}\right)\)

= \(\frac{1}{2}.\frac{10}{39}\)

=  \(\frac{5}{39}\)

Vậy kq = \(\frac{5}{39}\)

A