Đáp án:
B = $\frac{260}{87}$
Giải thích các bước giải:
B = $\frac{36}{1.3.5}$ + $\frac{36}{3.5.7}$ +...+ $\frac{36}{25.27.29}$
$= 9 . ( \frac{4}{1.3.5} +\frac{4}{3.5.7} +...+\frac{4}{25.27.29} )$
$= 9 .(\frac{1}{1.3} - \frac{1}{3.5} + \frac{1}{3.5} - \frac{1}{5.7} +...+ \frac{1}{25.27} - \frac{1}{27.29})$
$=9 .( \frac{1}{1.3} - \frac{1}{27.29} )$
$=9 . \frac{260}{783}$
$= \frac{260}{87}$
vậy B = $\frac{260}{87}$